Sphene stone beads

Sphene is one of the very few gemstones known for having a higher dispersion rating than fine diamond. A high dispersion rating results in remarkable fire, brilliance and scintillation. The attractive green color of sphene is often accompanied by golden tones. Sphene stone beads is available in reasonably large sizes, often weighing 5 carats or more. It is sometimes traded as 'titanite', a name derived from its titanium content. Sphene is also strongly pleochroic, which means that typically at least 3 colors can be seen in a single specimen depending on the angle from which it is viewed. Sphene is rather soft (5 to 5.5 on the Mohs scale), so its use in jewelry should be limited to pendants, earrings or brooches.

Grossularite Garnet natural gemstone beads

The garnets are differently colored minerals all with a common crystal structure and varying (but related) chemical compositions, with members ranging from the common red almandite to the rare green tsavorite and demantoid. Six common varieties of garnet are recognized based on their chemical composition. They are pyrope, almandine, spessartite, grossularite, uvarovite and andradite.

Grossularite garnet is a calcium-aluminum garnet. The name grossularite is derived from the botanical name for the gooseberry, grossularia. The grossularite group includes the light to medium green grossularite; the cinnamon-colored hessonite; the colorless leuco garnet; a dense opaque green garnet called hydrogrossular; and the rare and valuable deep green tsavorite garnet, colored by chromium. Grossularite garnet ranges in color from lemon yellow to greenish-yellow, yellowish-green and even mint green.

Grossularite varies in hardness from 6.5 to 7.5 on the Mohs hardness scale. The best specimens of grossularite can easily be confused with the more expensive demantoid garnet. Garnet's good hardness combined with its absence of cleavage makes it a very durable natural gemstone beads for all kinds of jewelry.

Deposits of grossularite are found in a number of locations, including Canada (Quebec), the USA (Vermont), Africa (South Africa, Tanzania, Kenya and Mali), Russia, Pakistan and Sri Lanka. 

Type I quartz colored varieties gemstone - Color caused by color center

"Rock Crystal" is the name for clear, colorless pure quartz crystal. Quartz is the second most abundant mineral in the Earth's continental crust with an overall formula SiO₂. Rock crystal can be found in some form in any geological environment that allows the formation of quartz in general. Large, well-formed and transparent crystals can be found. For nice crystals to form, conditions in a geological environment need to change more or less gradually and slowly over a long period of time.  A single rock crystal may have grown over a period of several million years during the uplift and folding of the mountains.

There are many different varieties of quartz, common colored varieties include citrine, rose quartz, amethyst, smoky quartz, blue quartz, and others, which are semi-precious gemstones. The color formation of rock crystal’s varieties will be reviewed as following.

Amethyst has been known as the most highly prized form of quartz. The color can vary from a pale purple to a dark purple. Very often the color is unevenly distributed and is most intense at the tips of the crystals.  Amethyst owes its color to the presence of iron built into its crystal lattice. Some of this iron sits in sites normally occupied by silicon and some is interstitial. The iron impurity is usually in the +3 valence state to substitute Si.  For natural amethyst, Gamma ray radiation from nuclear decay in the surrounding rocks can make charge transfer from an iron lattice site to an interstitial iron, and form +4 valence state of iron (purple color centers). This +4 iron absorbs certain wavelengths (357 and 545 nanometers) of light causing the purple color. It is needed to have quartz that contains the right amounts of iron and then is subjected to enough natural radiation to cause the color centers to form. 

Citrine is the yellow to brownish-red variety of the Quartz.  Citrine’s yellow color is caused by quantities of Fe³⁺ impurities which form color center inside quartz’s lattice. However, Citrine is somewhat rare in nature.  Most citrines on the market have been heat treated from amethyst or smoky quartz. Natural Citrine is yellow to orange-yellow, and occurs in much lighter hues than the heat-treated material, which is dark orange-brown to reddish-brown. Virtually all heat-treated material has a reddish tint, whereas the natural specimens do not.

Inexpensive amethyst is often heated at high temperatures to produce the more profitable orange yellow citrine. Amethyst has been known has iron impurities at +4 valence state.  Iron +4 valence state related purple color centers are not as stable as iron +3 related yellow color centers at high temperature, thus at high temperature only more stable yellow color centers exist to produce yellow color.  Citrines whose colors have been produced by artificial means tend to have much more of an orange or reddish caste than those found in nature, which are usually a pale yellow.  In some Amethyst deposits, the Amethyst has been partially or fully changed over to yellow Citrine by natural means of heating. It is relatively well known that the vast majority of citrine quartz is the product of heat treating amethyst. Material from Brazil and Uruguay has often been used for this purpose. Both large and small amethyst-lined geodes are converted to citrine using simple low-temperature heat treatment in air. Citrine made by heating amethyst may be returned to a purple color by bombarding it with radiation which cause color centers transfer back to purple color centers.

Ametrine contains both amethyst and citrine in contact with one another.  The colors of amethyst and citrine are produced by iron impurities with different oxidation states within the quartz.  Purple is thought to be produced by Fe⁴⁺ related color centers, of which the Fe impurities are oxidized to Fe⁴⁺ by natural radiation emitted in nearby rocks. The golden-yellow is thought to be produced by Fe³⁺ related color centers. The different oxidation states occur due to there being a temperature gradient across the crystal during its formation and by natural radiation.  The concentration of iron is much higher in the yellow than in the purple sectors. If heating up a natural Ametrine, only the purple sectors loose their color, and the result is a crystal with 3 almost colorless and 3 yellow sectors. 

The color of smoky quartz is caused by irradiation and aluminum impurities built into its crystal lattice to form the color-centers.  In normal geological environments, the color centers formation process can only take place at temperatures below 50°C, otherwise the rate of color center destruction surpasses that of color center formation.  Thus the color of smoky quartz crystals appeared long after the crystals have grown. It is estimated that it takes several million years for a crystal to assume a deep color by natural radiation emitted in nearby rocks.

Pink quartz’s color is caused by small amounts of aluminum, Al⁽⁺³⁾, and phosphorus, P⁽⁺⁵⁾, built pairwise into the crystal lattice to replace Si⁽⁺⁴⁾, and subsequent high energy irradiation. The two electrically neutral SiO₄ groups are replaced by one AlO₄^- and one PO₄⁺ group. Pink quartz is often accompanied by phosphate minerals, like (Mn,Fe)Al[(OH)₂|PO₄]•H₂O.  Many pink quartz is translucent to transparent. Pink quartz is very sensitive to light and will pale quickly in direct sunlight. This is an indication that the color is due to color centers whose formation appears to be triggered by high energy radiation.

Colored Quartz is an affordable gemstone, and is used in various form of jewelry.  High quality colored Quartz is usually faceted for ring and pendant centerpieces, and less quality colored Quartz is most often used in gemstone beads with different shapes:  round, coin, oval, square, pillow, cube, tube, et al. With colored Quartz beads in your favorite color and shape, you even can create your own unique piece of colored Quartz jewelry, and with plenty of opportunity to customize to fit your unique style. There are plenty of low-cost colored Quartz beads available through online store and be sure to check out the discounted colored Quartz beads first to start your design.

Citrine and its relation with amethyst gemstone

Citrine is the yellow to brownish-red variety of the Quartz. Quartz is a chemical compound of silicon and oxygen, silicon dioxide SiO₂, commonly called silica. If pure, quartz is a colorless, transparent, and very hard crystalline material of glass-like look.  Citrine’s yellow color is caused by quantities of Fe³⁺ impurities which form color center inside quartz’s lattice. However, Citrine is somewhat rare in nature.  Most citrines on the market have been heat treated from amethyst or smoky quartz. Natural Citrine is yellow to orange-yellow, and occurs in much lighter hues than the heat-treated material, which is dark orange-brown to reddish-brown. Virtually all heat-treated material has a reddish tint, whereas the natural specimens do not.

Inexpensive amethyst is often heated at high temperatures to produce the more profitable orange yellow citrine [1][2]. Amethyst has been known has iron impurities at +4 valence state.  Iron +4 valence state related purple color centers are not as stable as iron +3 related yellow color centers at high temperature, thus at high temperature only more stable yellow color centers exist to produce yellow color.  Citrines whose colors have been produced by artificial means tend to have much more of an orange or reddish caste than those found in nature, which are usually a pale yellow.  In some Amethyst deposits, the Amethyst has been partially or fully changed over to yellow Citrine by natural means of heating. It is relatively well known that the vast majority of citrine quartz is the product of heat treating amethyst. Material from Brazil and Uruguay has often been used for this purpose. Both large and small amethyst-lined geodes are converted to citrine using simple low-temperature heat treatment in air. Citrine made by heating amethyst may be returned to a purple color by bombarding it with radiation which cause color centers transfer back to purple color centers.

Some smoky quartz can be turned into citrine by careful heating at a high temperature [3][4][5].  The color of smoky quartz is caused by irradiation and traces of aluminum built into its crystal lattice. Aluminum replaces silicon to form a [AlO₄]^- group instead of [SiO₄].  Since the yellow color centers are often more stable than the smoky color centers, thus some smoky quartz can be turned into citrine by careful heating.  This type of citrine is colored by aluminum-based and irradiation-induced color centers related to those found in smoky quartz. Citrine produced by heat-treating Smoky Quartz sometimes has a "smoky" hue to it, and can be border lined between Citrine and Smoky Quartz, with either definition being correct.

Natural Citrine is not common and occurs sparingly in many large Quartz deposits. Most commercial gem-grade material comes from Rio Grande do Sul and Minas Gerais, Brazil, but almost all of the Brazilian material is heat treated Amethyst.
A classic exhausted locality for natural Citrine is Olkhovka in the Northern Ural Mountains, Russia. Other locations where natural Citrine is found is San Cristobal, Santander, Colombia; Lubumbashi in Katanga (Shaba), Congo (Zaire); Antananarivo Province, Madagascar; Salamanca, Spain; and Dauphine, France.

There are not many yellow gemstones in the world of jewels. A diamond or a sapphire or a topaz may be yellow - those will be expensive. However, the citrine fulfills everyone's color wishes, from lemon yellow to reddish brown. Citrine is often used as an inexpensive substitute for Topaz.

High quality Citrine is usually faceted for jewelry and less quality Citrine is most often used as gemstone beads with different shapes:  round, coin, oval, square, pillow, cube, tube, et al.

Reference:

[1] Chudoba, K. F. (1962): Some relations between the causes of amethyst, smoky quartz, and citrine colors as given by modern science. Mineralogicheskii Sbornik (Lvov), (16), 91-105.

[2] Lehmann, G. (1972): Yellow color centers in natural and synthetic quartz. Physik der Kondensierten Materie 13, 297-306.

[3] Maschmeyer, D. et al (1980): Two modified smoky quartz centres in natural citrine. Phys.Chem.Minerals (6), 145-156

[4] Schmetzer, K. (1988): Thermal stability of yellow color centers in natural citrine. Neues Jahrbuch für Mineralogie, Monatshefte 2, 71-80.

[5] Rossman, G.R. (1994): Colored varieties of the silica minerals. In: Reviews in Mineralogy, Vol.29, Silica - Physical behavior, geochemistry and materials applications, Mineralogical Society of America.

Aventurine gemstone and color formation

Aventurine is a variety of translucent poly-crystal quartz with inclusions of minerals.  It is fundamentally different from Amethyst which is also a variety of quartz with the presence of manganese and iron in quartz, the manganese and iron are impurities in atomic level to form new energy states (color centers) that produces the purple color. In Aventurine case, the inclusions of minerals are much larger particles which reflect light that enters the quartz and produce a sparkly appearance.  The common color of aventurine is green and red orange, but it may also be brown, yellow, blue, or purple.

Green aventurine is a variety of translucent poly-crystal quartz with inclusions of Fuchsite, also known as chrome mica, is a chromium (Cr) rich variety of the mineral muscovite, belonging to the mica group of phyllosilicate minerals, with the following formula: K(Al,Cr)₂(AlSi₃O₁₀)(OH)₂, which display apple green color.  Bright reflection light from apple green Fuchsite inclusions inside quartz produce a sparkly silvery green appearance for green aventurine.

For red orange and brown aventurine, the colors are attributed to inclusions of red orange hematite and brown hematite inside quartz.  Hematite is the mineral form of iron oxide (Fe₂O₃), colored black to steel or silver-gray, brown to reddish brown, or red.  Bright reflection light from red orange hematite or brown hematite inclusions inside quartz produce a sparkly red orange and brown appearance for red orange and brown aventurine.

For yellow aventurine, the colors are attributed to inclusions of Pyrite. The mineral pyrite, or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula FeS₂. This mineral's metallic luster and pale brass-yellow hue give it a superficial resemblance to gold, hence the well-known nickname of fool's gold. Bright reflection light from golden Pyrite inclusions inside quartz produce a sparkly yellow appearance for yellow aventurine.

For blue aventurine, the colors are attributed to inclusions of blue Dumortierite . Dumortierite is a fibrous variably colored aluminium boro-silicate mineral, Al₇BO₃(SiO₄)₃O₃.  The crystals are varied in color from brown, blue, and green to more rare violet and pink. Substitution of iron and other tri-valent elements for aluminium result in the color variations. Bright reflection light from blue Dumortierite inclusions inside quartz produce a sparkly blue appearance for blue aventurine.

For purple aventurine, the colors are attributed to inclusions of purple Lepidolite.  Lepidolite is a lilac-gray to rose-colored member of the mica group with formula K(Li,Al,Rb)₃(Al,Si)₄O₁₀(F,OH)₂. Bright reflection light from purple Lepidolite inclusions inside quartz produce a sparkly purple appearance for purple aventurine.

The majority of green and blue-green aventurine originates in India. Creamy white and red orange material is found in Chile, Spain and Russia.

Aventurine is a popular gemstone because of its beauty and also because it is not highly priced.  Aventurine is formed into gemstone beads for jewelry making.

Aquamarine gemstone beads & coloration

Aquamarine is an affordable gemstone belonging to the Beryl family (chemical formula Be₃Al₂(SiO₃)₆). Pure Beryl is colorless.  However, some impurities cause the diverse amount of colors and many varieties. The pale blue color of aquamarine is attributed to iron impurities’ ions Fe²⁺.  The Fe³⁺ ions produce golden-yellow color, and when both Fe²⁺and Fe³⁺ are present, the color is a darker blue due to Fe²⁺-O-Fe³⁺ inter-valence charge transfer.  Aquamarine ranges in color from a faint light blue to blue and bluish-green, due to the concentration of iron impurities and its ions valence state, with lighter colored stones being the more common type.

The green hues in most Aquamarine can be removed through heat treatment. Its color fades to white when exposed to sunlight or is subjected to heat treatment. Natural Aquamarine is usually lighter and greener in color, and heat treatment creates deeper bluer hues due to change of iron impurities’ ions valence state.

Aquamarine has good hardness (7.5 to 8 on the Mohs scale), the hardness makes it very tough and protects it to a large extent from scratches, but it may develop internal cracks if banged hard.  Aquamarine is belonging to hexagonal crystal group, its moderate refraction and its weight in common with the other members of Beryl family.

Aquamarine gemstone is mined mainly in Brazil, but also is found in Nigeria, Madagascar, Zambia, Pakistan, and Mozambique. In the United States, aquamarines can be found at the summit of Mt. Antero in the Sawatch Range in central Colorado. In Wyoming, aquamarine has been discovered in the Big Horn Mountains.

The largest aquamarine of gemstone quality ever mined was found in Marambaia, Minas Gerais, Brazil, in 1910. It weighed over 110 kg, and its dimensions were 48.5 cm x 42 cm.

High quality flawless single crystal aquamarine is cut to form gemstone beads to maximize the intensity of its color, and to display cat's eye effect and asterism.  Lesser quality poly-crystal aquamarines lacking good transparency are formed into stone beads with different shapes.

Ametrine gemstone beads color formation

Ametrine contains both amethyst and citrine in contact with one another.  The colors of amethyst and citrine are produced by iron impurities with different oxidation states within the quartz [1].  Purple is thought to be produced by Fe⁴⁺ related color centers, of which the Fe impurities are oxidized to Fe⁴⁺ by natural radiation emitted in nearby rocks. The golden-yellow is thought to be produced by Fe³⁺ related color centers. The different oxidation states occur due to there being a temperature gradient across the crystal during its formation and by natural radiation.  The concentration of iron is much higher in the yellow than in the purple sectors. If heating up a natural Ametrine, only the purple sectors loose their color, and the result is a crystal with 3 almost colorless and 3 yellow sectors.

If a well-formed Ametrine crystal is sawn perpendicular to the c-axis, the color zones of amethyst and citrine often form a geometric pattern that radiates outwards from the c-axis like the pieces of a pie. Straight-line contacts separate zones of amethyst from zones of citrine.  These crystals usually contain zones of clear quartz, amethyst and citrine. When these crystals are cut into pieces that are appropriately sized for faceting gemstones, only a portion of the stones will be Ametrine. The remainder will be amethyst, citrine, and clear quartz [2].

Laboratory experiments in 1981 determined that heat and irradiation can be used to convert natural amethyst into a bicolor material that has an appearance similar to natural Ametrine [4].  Since 1994, a Russian laboratory has perfected the industrial production of bicolored quartz crystals that are later irradiated to bring out the typical Ametrine colors [3] to form Ametrine gemstone beads for jewelry.

Like all crystal quartzes, the Ametrine has a hardness of 7 on the Mohs scale and is thus insensitive to scratches.

Ametrine is rarely found in nature. The only known and well confirmed location for Ametrine is an area in Bolivia near the Brazilian border [2]. It's commercially exploited and almost all of the world's commercial natural Ametrine production has been from the Anahi  Mine in southeastern Bolivia.

High quality Ametrine is usually faceted for jewelry and less quality Ametrine is most often used as stone beads with different shapes:  round, coin, oval, square, pillow, cube, tube, et al. With Ametrine beads in your favorite color and shape, you even can create your own unique piece of Ametrine jewelry, and with plenty of opportunity to customize to fit your unique style. There are plenty of low-cost Ametrine beads available through online store and be sure to check out the discounted Ametrine beads first to start your design.

Reference:

[1] G.R. Rossman, “Colored varieties of the silica minerals”, Reviews in Mineralogy, Vol.29 

[2] Vasconcelos P, Wenk HR, Rossman GR (1994) The Anahí ametrine mine,
Bolivia. Gems and Gemology 30, 4-23.

[3] Balitsky VS, Lu T, Rossman GR, Makhina IB, Mar'in, AA, Shigley JE, Elen S,
Dorogovin BA (1999) Russian synthetic ametrine. Gems and Gemology 35,
122-134.

[4] Balitsky, V. S.; Machina, I. B.; Mar'in, A. A.; Shigley, J. E.; Rossman, G. R.; Lu, T. (2000): Industrial growth, morphology and some properties of Bi-colored amethyst-citrine quartz (ametrine). Journal of Crystal Growth 212, 255-260.

What cause Amazonite stone beads blue color

Amazonite is a popular gemstone usually varies in color from yellow-green to blue green and may also exhibit fine white streaks.  It was named after Amazon River, from which certain green stones were formerly obtained, however it is doubtful whether Amazonite occurs in the Amazon area. 

Amazonite is green to blue-green variety of Microcline.  Microcline (KAlSi₃O₈) may be clear, white, pale-yellow, brick-red, or green color depends on the impurities which contain. Common Impurities inside Microcline are Fe, Ca, Na, Li, Cs, Rb, H₂O, Pb.   The source of amazonite’s color was uncertain for years, and many assumed the color was due to copper, which often produces blue and green colors in gems and minerals.  However, studies [1] suggest that the blue-green color caused by an elevated content of Pb and water in the Microcline. In the past two to three decades, some inconsistencies with this possible correlation have become apparent.  Analyses by Hoffmeister and Rossman (1985) determined that some samples of green amazonite did indeed have a high lead concentration, whereas some non-green samples of microcline also had unexpectedly high lead concentrations. They propose that while lead does indeed play a role in coloring amazonite, both natural radiation and structural water are necessary to produce monovalent or trivalent lead for color of amazonite. Therefore the color of amazonite could possibly be due to three variables: lead, water and a form of ionizing radiation.  More recent studies [2] suggest that the green color would arise from the ion Pb⁺, whereas the blue one would be attributed to the ion Pb³⁺. Thus the blue-green color is due to mixture of Pb⁺ and Pb³⁺.

Amazonite often has white lines or alternating streaks mixed in, and can have uneven color distribution. A deep forest-green color is most preferred, but Amazonite gemstones can also be light green and bluish-green.  Grayish-green and very faint green stones also exist, but are not commonly used as gemstones.  

Sunlight can sometimes enrich the color of genuine amazonite.  Amazonite’s color is lost on heating to over 300°C and can be restored by irradiation if heating was insufficient (<500°C) to cause water loss. It has a hardness range of 5 to 6 on the Mohs Scale.  Care should be handled with Amazonite as it is sensitive to pressure and can easily crack or chip. 

Because of its bright green or blue-green color when polished, amazonite is sometimes cut and used as gemstone beads, although it is easily fractured.  You will not see amazonite very often in the current market. It makes some very pleasing jewelry items but is kind of hard to find.

Amazonite is an inexpensive gemstone is most often used in stone beads with different shapes:  round, coin, oval, square, pillow, cube, tube, et al. With Amazonite beads in your favorite color and shape, you even can create your own unique piece of Amazonite jewelry, and with plenty of opportunity to customize to fit your unique style. There are plenty of low-cost Amazonite beads available through online store and be sure to check out the discounted Amazonite beads first to start your design.

Reference:

[1] Brightwell, Stephanie, 1999; "Coloration due to Lead Levels in Blue microcline Feldspar (Amazonite) from the Morefield Pegmatite, Amelia, Virginia",

[2] Julg, A. (1998): A theoretical study of the absorption spectra of Pb+ and Pb3+ in site K+ of microcline: application to the color of amazonite. Physics and Chemistry of Minerals 25, 229-233. 

Amethyst gemstone color formation

Amethyst has been known as the most highly prized form of quartz. The color can vary from a pale purple to a dark purple. Very often the color is unevenly distributed and is most intense at the tips of the crystals.  Amethyst owes its color to the presence of iron built into its crystal lattice [3]. Some of this iron sits in sites normally occupied by silicon and some is interstitial. The iron impurity is usually in the +3 valence state to substitute Si.  For natural amethyst, Gamma ray radiation [1] [2] from nuclear decay in the surrounding rocks can make charge transfer from an iron lattice site to an interstitial iron, and form +4 valence state of iron (purple color centers). This +4 iron absorbs certain wavelengths (357 and 545 nanometers) of light causing the purple color. It is needed to have quartz that contains the right amounts of iron and then is subjected to enough natural radiation to cause the color centers to form. 

The most famous and commercially important amethyst locations are in volcanic rocks. Amethyst is also found in igneous, and certain metamorphic rocks, as those contain enough radioactive trace elements for a sufficient irradiation of the crystals.  As the iron is built into the crystal lattice, amethyst is sometimes found in ore deposits. Sometimes amethyst banded with purple and white lines. 

Amethyst is crystalized Silicon Dioxide belong to hexagonal crystal group, its moderate refraction and its weight in common with the other quartzes. Amethyst’s Hardness is 7, and can be melted at 1650 degree and is insoluble in common solvents. It has been known that the amethyst changes its color on being heated above 250 degree due to charge transfer from iron +4 valence states to +3 valence states, and the color become darker under irradiation due to more charge transfer from iron +3 valence states to +4 valence states.

Synthetic amethyst can be produced by irradiation of clear quartz to form iron +4 valence state (purple color centers), and the clear quartz has been first doped with iron impurities. On exposure higher temperature above 250 degree, the irradiation effects can be partially cancelled and the amethyst generally becomes yellow or even green.

Amethyst is a very common mineral and is found worldwide. The deposits with the greatest economic significance are in various states in southern Brazil and in Uruguay.  The third major export country is Madagascar. Good amethyst was found in Aztec graves, though the deposits from which they were extracted are no longer known today. On the Canadian side of Lake Superior in North America, there is a place named Amethyst Harbor. The violet quartz is found there in ample quantities, though rarely in gemstone quality.

Brilliant purple amethyst gemstone beads are an eye-catching choice for jewelry. Romantic and passionate amethyst, the traditional birthstone for those born in February, is said to represent courage and inner strength. Amethyst gemstone jewelry makes a thoughtful and personal birthday gift.

High quality amethyst is cut to maximize the intensity of its color. Amethyst is also formed into stone beads with different shapes: round, coin, oval, square, pillow, cube, tube, et al. With amethyst beads in your favorite color and shape, you even can create your own unique piece of amethyst jewelry, and with plenty of opportunity to customize to fit your unique style. There are plenty of low-cost amethyst beads available through online store and be sure to check out the discounted amethyst beads first to start your design.

Do not expose an amethyst to direct sunlight for a long time. Very likely it will pale out by the ultraviolet radiation. Some amethyst pales out really quickly and some very slowly, but you can't tell in advance. Thus amethyst jewelry should not be worn while sunbathing, and sudden changes of temperature can also be harmful to the amethyst.

[1] G.R. Rossman, “Colored varieties of the silica minerals”, Reviews in Mineralogy, Vol.29 

[2] Nassau, K. (1975a) The origins of color in minerals and gems. Lapidary J., 29, 920-8, 1060-70, 1250-8, 1521.

[3] E.F. Holden, The cause of color in smoky quartz and amethyst, American Mineralogist, Vol.9, 203-252, 1925