Type II quartz colored varieties gemstone - Color caused by inclusions

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.

For Rose quartz, in spite of extensive studies over the last century, the source of the rose coloration in quartz still remains a subject of active debate. Many possible mechanisms for the color of rose quartz have been proposed, but no consensus has developed. More recently it has been suggested  that fibrous inclusions are responsible for color of rose quartz. The nature of the colorizing nanofibers in rose quartz has recently been studied, following the more "direct" approach that was already chosen, they dissolved rose quartz from various locations in hydrofluoric acid and extracted mats of visually homogeneous, fine textured, flaky pink-colored nanofibers.  The fibrous mineral inclusions are rose-colored mineral and could be a mixture of dumortierite and a related phase. The mineral makes up only about 0.05% - 0.15% of the overall weight of rose quartz. The mineral will bleach when heated above 500°C, but will not regain its color when irradiated. Color loss can be induced by oxidizing agents, too, and heat treatment in a reducing atmosphere will turn heated and paled material rose-colored again.  Experiments also show the embedded mineral fibers are not randomly oriented, but preferrably oriented along specific crystallographic axes.  Thus pink-colored fibrous inclusions cause the coloration of rose quartz.

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.

There are three type of Blue quartz: Type I: Blue color of single crystals quartz is due to Rayleigh scattering effect.  The Rayleigh scattering is the scattering of light by particles much smaller than the wavelength of the light.  It is the scattered light by small particles in the atmosphere that gives the blue sky. In blue quartz case, the light is scattered by abundant submicrometer-sized inclusions inside quartz crystal to cause sky blue color. The total amount of inclusions in the blue quartz is calculated to be 0.02 %. This is the rarest form of blue quartz, and this type of blue quartz exhibits a particularly beautiful blue coloration, which is sky blue in the center of each crystal and darker at the margins.  In addition, in certain orientations these crystals display an intense silver-blue chatoyant flash.

Type II: Blue color of single crystal quartz is due to evenly distributed inclusions of blue minerals, like magnesio-riebeckite or tourmaline. The blue color is caused by light reflection  of the included blue minerals, however the inclusions are individually too large (~0.1 x 1 x 20 μm) to contribute to the blue color by Rayleigh scattering.

Type III: Blue color of ploy-crystal quartz is due to inclusions of blue minerals, the inclusions of blue minerals have much larger size than the size of inclusions in type I and type II.  This type of blue quartz is similar to blue aventurine that is also colored by embedded blue minerals, like dumortierite. In general, blue quartz has less blue inclusions and shows light blue color with blue pattern.  Blue aventurine has more blue inclusions and shows evenly distributed dark blue color. There is no clear boundary between type III blue quartz and blue aventurine.

Rutilated quartz is a variety of Quartz with needle-like rutile inclusions, embedded in it. The rutile needles can be reddish, golden or silvery.

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 for jewelry design.