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.