Zoisite

Zoisite

is a mineral that includes several gem varieties. The most important and well-known is Tanzanite, a sensational blue gemstone. The variety Thulite is used as a minor pink gemstone. A variety known as Ruby Zoisite, which is green Zoisite associated with opaque red Ruby (and often black amphibole streaks), is used as a carving gemstone as well as a minor gem. The term Zoisite is rarely used on its own in the gemstone trade.Tanzanite and Thulite each have their own dedicated page. 

Chemical Formula	Ca2Al3(SiO4)3(OH)
Color	Blue, Red, Green, Pink, Purple
Hardness	6Â -Â 6.5
Crystal System	Orthorhombic
Refractive Index	1.69Â -Â 1.70
SG	3.2Â -Â 3.4
Transparency	Opaque. May be translucent in thin backlit sections.
Double Refraction	.009
Luster	Vitreous
Cleavage	1,1
Mineral Class	Zoisite

Zircon

Zircon

is an important gemstone of many colors, and is an historical gemstone used for thousands of years. It is sometimes looked upon as a cheap Diamond simulant, but in actuality it can be a valuable gem. Its color diversity is caused by traces of certain impurities, some of which are radioactive. Zircon gemstones containing radioactive element traces undergo a process called metamiction, in which their inner crystal structure becomes destroyed. These radioactive forms of Zircon must be heated to stabilize them for use as gems. In fact, many of the gem forms of zircon (even those that are not radioactive) are heat treated to enhance color and increase transparency. The radioactivity in Zircon gemstones is very minimal and is generally believed to be safe, without posing any health hazards.

Chemical Formula	ZrSiO4
Color	White, Colorless, Blue, Red, Green, Yellow, Orange, Brown, Pink, Purple, Gray
Hardness	7.5
Crystal System	Tetragonal
Refractive Index	1.930Â -Â 1.987
SG	4.6Â -Â 4.8
Transparency	Transparent
Double Refraction	.059
Luster	Adamantine
Cleavage	3,2
Mineral Class	Zircon

Variscite

Variscite

is an opaque gemstone with interesting color splotches and veins. It is usually a light or minty-green with whitish patterns. Almost all Variscite gemstones originate from Utah, and for this reason it may also be called Utahlite. In fact, the most important deposit of gemstone-grade Variscite is actually called the Utahlite Deposit.

Chemical Formula	AlPO4 · 2(H2O)
Color	Green
Hardness	3.5Â -Â 4.5
Crystal System	Orthorhombic
Refractive Index	1.55Â -Â 1.59
SG	2.5Â -Â 2.6
Transparency	Opaque. May be translucent when backlit on the edges.
Double Refraction	.016
Luster	Waxy
Cleavage	1,1;2,1
Mineral Class	Variscite

Uvarovite

Uvarovite

is the rarest of the familiar Garnets, and is seldom used as a gem. The term Garnet describes a group name for several closely related minerals that form important gemstones, and Uvarovite is an individual member mineral of the Garnet group. It only occurs in very small crystals, and crystals are rarely large enough for faceting. The color of Uvarovite is a deep chrome-green, and Uvarovite is a unique Garnet being that it only comes in one color.

Chemical Formula	Ca3Cr2Si3O12
Color	Green
Hardness	6.5Â -Â 7
Crystal System	Isometric
Refractive Index	1.860Â -Â 1.870
SG	3.7Â -Â 3.8
Transparency	Transparent to translucent
Double Refraction	None
Luster	Adamantine
Cleavage	None
Mineral Class	Uvarovite (Garnet)

Unakite

Unakite

is a metamorphic rock primarily composed of pistachio green epidote and pinkish orthoclase, named after a mountainous area in southern United States where it was found. Unakite is colorful and expressive in appearance and is often used in design applications such as flooring and ornament decor. Unakite is a form of orthoclase feldspar, and the orthoclase structure has the luster of moonstone, set in the particle structure of the epidote.

Chemical Formula	KAlSi3O8
Color	Green, Yellow, Pink, Gray
Hardness	6Â -Â 6.5
Crystal System	Monoclinic
Refractive Index	1.52Â -Â 1.53
SG	2.56Â -Â 2.59
Transparency	Transparent
Double Refraction	-.008
Luster	Vitreous to rsinous
Cleavage	2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal
Mineral Class	Sanidine and Silicate

Turquoise

Turquoise

is an ancient gemstone, and has been highly regarded in many of the world"s civilizations. Among the ancient empires it is best known among the Persian and Native American civilizations, where it was the most popular ornamental gemstone. Turquoise today remains a popular gemstone, and is one of the most important opaque gemstones. It is highly regarded due to its unique turquoise-blue color, and is the only gemstone to have an exclusive color named after it.

Chemical Formula	CuAl6(PO4)4(OH)8 • 4H2O
Color	Blue, Green, Multicolored
Hardness	5Â -Â 6
Crystal System	Triclinic
Refractive Index	1.61Â -Â 1.65
SG	2.6Â -Â 2.8
Transparency	Opaque
Double Refraction	.04
Luster	Waxy
Cleavage	None
Mineral Class	Turquoise

Tsavorite

Tsavorite

is a trade name for the emerald-green variety of Grossular Garnet that originates in Africa. Tsavorite has become one of the most popular and expensive Garnets, due to its rarity combined with effective marketing tactics. This gemstone was first discovered in Tanzania in 1967. It was subsequently found in the same geological formation across the border in Kenya in 1970 near Tsavo National Park, a wildlife preserve in the African Serengeti, and given its name after the national park. Tsavorite was named by both Campbell Bridges, the discoverer of Tsavorite, and by Henry Platt, the president of Tiffany & Co. (one of the world"s largest jewelry companies).

Chemical Formula	Ca3Al2Si3O12
Color	Green
Hardness	7Â -Â 7.5
Crystal System	Isometric
Refractive Index	1.738Â -Â 1.745
SG	3.6
Transparency	Transparent
Double Refraction	None
Luster	Vitreous to Adamantine
Cleavage	None
Mineral Class	Grossular (Garnet)

Tourmaline

Tourmaline

is the most colorful of all gemstones. It occurs in all colors, but pink, red, green, blue and multicolored are its most well-known gem colors. Scientifically, tourmaline is not a single mineral, but a group of minerals related in their physical and chemical properties. The mineral Elbaite is the member of the Tourmaline group that is responsible for almost all the gem varieties. Three other members of the group - Schorl, Dravite and Liddicoatite, are seldom used as gemstones.

Chemical Formula	Tourmaline is a series of several different minerals with unique chemical formulas. See The chemical formula of Tourmaline for details.
Color	White, Colorless, Blue, Red, Green, Yellow, Orange, Brown, Pink, Purple, Gray, Black, Multicolored
Hardness	7Â -Â 7.5
Crystal System	Hexagonal
Refractive Index	1.616Â -Â 1.650
SG	2.9Â -Â 3.3
Transparency	Transparent to opaque
Double Refraction	.018
Luster	Vitreous
Cleavage	3,2
Mineral Class	Tourmaline

Topaz

Topaz

makes an ideal gem. A good hardness and desirable colors, combined with a relative abundance and availability makes it one the most popular gemstones. The most valuable colors of Topaz are the golden orange-yellow type, called Imperial Topaz, and the dark pinkish-red and orange-red colors. Value increases with a deepness of color in orange and reddish hues. The most commonly used colors of Topaz in jewelry are the blue types. It was not until this past century that blue Topaz became widespread on the gem market, since virtually all blue gem Topaz is irradiated and heat treated.

Chemical Formula	Al2SiO4(F,OH)2
Color	White, Colorless, Blue, Red, Green, Yellow, Orange, Brown, Pink, Purple, Gray, Multicolored
Hardness	8
Crystal System	Orthorhombic
Refractive Index	1.610Â -Â 1.638
SG	3.4Â -Â 3.6
Transparency	Transparent
Double Refraction	.014
Luster	Vitreous
Cleavage	1,3 - basal
Mineral Class	Topaz

Tiger"s Eye

Tiger"s Eye

, a popular yet inexpensive gemstone, is a pseudomorph of compact Quartz after the fibrous mineral Crocidolite. It is formed when the Quartz takes over and dissolves the Crocodolite, leaving the Quartz in a finely fibrous and chatoyant form. When polished, it"s silky luster creates a beautiful chatoyant effect of moving layers of brown and yellow lines and waves. 

Chemical Formula	SiO2
Color	Yellow, Brown, Multicolored
Hardness	7
Crystal System	Hexagonal
Refractive Index	1.54Â -Â 1.55
SG	2.63Â -Â 2.65
Transparency	Opaque
Double Refraction	None
Luster	Silky
Cleavage	Indiscernible
Mineral Class	Quartz (Chalcedony)

Thulite

Thulite

is the pink to reddish variety of the mineral Zoisite. It is an opaque stone composed of dense crystals, and is used as a minor gemstone.

Chemical Formula	Ca2Al3(SiO4)3(OH)
Color	Red, Pink
Hardness	6Â -Â 6.5
Crystal System	Orthorhombic
Refractive Index	1.69Â -Â 1.70
SG	3.2Â -Â 3.4
Transparency	Opaque. May be translucent in thin backlit sections.
Double Refraction	.009
Luster	Vitreous
Cleavage	1,1
Mineral Class	Zoisite

Tanzanite

Tanzanite is the blue to violet gem variety of the mineral Zoisite. Since its relatively recent discovery in 1967, it has become a mainstream and popular gemstone, and is used extensively in jewelry. To date, Tanzanite is found only in the Arusha region of the African country of Tanzania, and it was named after its country of origin.

Chemical Formula	Ca2Al3(SiO4)3(OH)
Color	Blue, Purple
Hardness	6Â -Â 6.5
Crystal System	Orthorhombic
Refractive Index	1.69Â -Â 1.70
SG	3.2Â -Â 3.4
Transparency	Transparent to translucent
Double Refraction	.009
Luster	Vitreous
Cleavage	1,1
Mineral Class	Zoisite

Sard

Sard

is a variety of Chalcedony cryptocrystalline in nature. This reddish tinted version derives its name from the ancient city Sardis. Mixed with white bands, the combined colors create a beautiful stone known as Sardonyx which has historically had very high value.

Chemical Formula	SiO2
Color	Red, Yellow, Orange, Brown, Pink, White, Gray, Black, Banded, Multicolored
Hardness	6.5Â -Â 7
Crystal System	Hexagonal
Refractive Index	1.54Â -Â 1.55
SG	2.63Â -Â 2.65
Transparency	Translucent to opaque
Double Refraction	.009
Luster	Vitreous to waxy
Cleavage	None
Mineral Class	Quartz (Chalcedony)

Sunstone

Sunstone

is an interesting feldspar gemstone with a glittery appearance and usually an orange or reddish color, hence its name. The glitter effect is caused by inclusions of the mineral Hematite, or sometimes also Goethite or Pyrite (and in one rare instance, Copper). The term used to describe the glittery effect exhibited on Sunstone is aventurescence.

Chemical Formula	Microcline: KAlSi3O8Oligoclase: (Na,Ca)Al1-2Si3-2O8
Color	Red, Orange, Brown, Pink
Hardness	6Â -Â 6.5
Crystal System	Triclinic
Refractive Index	1.525Â -Â 1.548
SG	2.62Â -Â 2.65
Transparency	Transparent to translucent
Double Refraction	0.01
Luster	Vitreous
Cleavage	2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal
Mineral Class	Microcline or Oligoclase

Spodumene

Spodumene

is a mineral that contains two important gem varieties: pink Kunzite and green Hiddenite. Both of these gemstone varieties have their own dedicated page. In the gem trade, the term Spodumene is generally used to include only the yellow, brownish, light greenish, and colorless forms of Spodumene.

Chemical Formula	LiAlSi2O6
Color	White, Colorless, Blue, Green, Yellow, Orange, Brown, Pink, Purple, Gray, Multicolored
Hardness	6.5Â -Â 7
Crystal System	Monoclinic
Refractive Index	1.66Â -Â 1.68
SG	3.1Â -Â 3.2
Transparency	Transparent
Double Refraction	.015
Luster	Vitreous
Cleavage	1,2 - prismatic
Mineral Class	Spodumene

Spinel

Spinel

is most famous for its deep red variety that closely resembles Ruby. These two gemstones can be very difficult to distinguish. Until the late 19th century, there was no distinction made between Ruby and red Spinel, as they look identical and are found in the same localities. Many famous old "Rubies" were discovered to be in fact Spinel. For example, the enormous "Ruby" that forms the centerpiece of the royal crown of England (the Black Prince"s Ruby) was actually determined to be Spinel.

Chemical Formula	MgAl2O4
Color	White, Colorless, Blue, Red, Green, Yellow, Orange, Brown, Pink, Purple, Black
Hardness	7.5Â -Â 8
Crystal System	Isometric
Refractive Index	1.712Â -Â 1.736
SG	3.5Â -Â 3.7
Transparency	Transparent to translucent
Double Refraction	None
Luster	Vitreous
Cleavage	None, but may exhibit slight parting
Mineral Class	Spinel

Sphene

Sphene is the gemstone name of the mineral Titanite. Sphene is known for its exceptional dispersion, resulting in gemstones that show brilliant fire. The dispersion of Sphene exceeds even that of Diamond. However, the relatively low hardness of Sphene inhibits its use as a mainstream gemstone.

Chemical Formula	CaTiSiO5
Color	Green, Yellow, Orange, Brown
Hardness	5Â -Â 5.5
Crystal System	Monoclinic
Refractive Index	1.84Â -Â 2.11
SG	3.4Â -Â 3.6
Transparency	Transparent
Double Refraction	.100 - .192
Luster	Adamantine
Cleavage	2,2
Mineral Class	Titanite

Spessartite

Spessartite

is an orange to orange-red form of the gemstone Garnet. The term Garnet describes a group name for several closely related minerals that form important gemstones, and Spessartite is an individual member mineral of the Garnet group. Spessartite is synonymous with Spessartine, though in the gem trade the name Spessartite is more frequently used, and in the mineral trade the term Spessartine is more frequently used. The term Spessartite is often identified together with the Garnet name and called Spessartine Garnet in the trade.

Chemical Formula	Mn3Al2Si3O12
Color	Red, Yellow, Orange, Brown, Pink
Hardness	7
Crystal System	Isometric
Refractive Index	1.795Â -Â 1.815
SG	4.1Â -Â 4.2
Transparency	Transparent
Double Refraction	None
Luster	Adamantine
Cleavage	None
Mineral Class	Spessartine (Garnet)

Sodalite

Sodalite

is a blue gemstone almost invariably veined with white streaks or markings. Its ideal color is an intense blue, and it comes in all shades of blue from light blue to deep royal-blue, and from grayish-blue to violet. Different shades of blue will often be present in a single gemstone. Sodalite is an opaque gemstone, though it is slightly translucent on thin edges.

Chemical Formula	Na8Al6Si6O24Cl2
Color	Blue, Purple
Hardness	5.5Â -Â 6
Crystal System	Isometric
Refractive Index	1.48
SG	2.2Â -Â 2.3
Transparency	Opaque
Double Refraction	None
Luster	Greasy or waxy
Cleavage	3,6
Mineral Class	Sodalite

Smoky Quartz

Smoky Quartz

is the brown "smoky" variety of Quartz. It ranges in color from light grayish-brown to deep black. Smoky Quartz can be opaque, but is almost always transparent to translucent, even when in its darkest color shade. Smoky Quartz is very common and was never an historically important gemstone. Only in very recent times has it become a popular gemstone.

Chemical Formula	SiO2
Color	Brown, Black
Hardness	7
Crystal System	Hexagonal
Refractive Index	1.54Â -Â 1.55
SG	2.63Â -Â 2.65
Transparency	Transparent to translucent
Double Refraction	.009
Luster	Vitreous
Cleavage	Indiscernible
Mineral Class	Quartz

Silver

Silver

is an important precious metal, and has been used by ancient civilizations throughout history as a second to Gold in importance and value. Like Gold, it has always been used as a monetary standard, and ancient silver ornaments and silverware dating back centuries have been found throughout the world. 

Chemical Formula	Ag
Color	Metallic, White
Hardness	2.5Â -Â 3
Crystal System	Isometric
SG	9.0Â -Â 12.0
Transparency	Opaque
Luster	Metallic
Cleavage	None
Mineral Class	Silver

Scapolite

Scapolite

is a common mineral, and its typical habit is in opaque and uninteresting forms. However, several unique localities provide gemmy transparent forms of this mineral, which make a rare and pretty gem. However, despite its lovely colors, Scapolite has been limited from becoming a more mainstream gemstone due to its relatively low hardness.

Chemical Formula	(Na,Ca)4(Al,Si)3Si6O24)(Cl,CO3)
Color	White, Colorless, Yellow, Orange, Brown, Pink, Purple
Hardness	5Â -Â 6
Crystal System	Tetragonal
Refractive Index	1.54Â -Â 1.58
SG	2.5Â -Â 2.8
Transparency	Transparent to opaque
Double Refraction	-0.006 to -0.0037
Luster	Vitreous
Cleavage	2,2
Mineral Class	Scapolite (Marialite / Meionite)

Sapphire

Sapphire is the name of a category of precious gemstones formed from the mineral class named corundum.[1]#3Corundum is high on the hardness chart [2], which typically translates to more valuable gemstones.

Sapphire is best known as a deep blue transparent stone having a distinct crystalline appearance. The blue color comes from the presence of impurities of titanium and iron. Both need to be present to create the dramatic blue presentation. [5] The most popular cuts for a sapphire are oval, round and cushion. [4]

Notes from Wikipedia
* In geology, the term mineral is usually reserved for mineral species: mineral species are crystalline compounds meaning they have well-defined chemical composition and a specific crystal structure.
* The International Mineralogical Association (IMA) is the premier global standards body for the definition and nomenclature of mineral species.

• References
• 1. Wikipedia
• 2. GemSociety.org
• 3. Here we are referring to corundum in the rock-forming sense.
• 4. FourMine.com
• 5. WebExhibits.com, Cause of Color (in gemstones)

.

Chemical Formula	Al2O3
Color	White, Colorless, Blue, Green, Yellow, Orange, Brown, Pink, Purple, Gray, Black, Multicolored
Hardness	9
Crystal System	Trigonal
Refractive Index	1.76Â -Â 1.77
SG	3.9Â -Â 4.1
Transparency	Transparent to opaque
Double Refraction	.0008
Luster	Vitreous to adamantine
Cleavage	None, but may exhibit parting
Mineral Class	Corundum

Rhodochrosite

Rhodochrosite has a lovely natural pink to red color. Rhodochrosite gemstones are primarily translucent to opaque streaked beads and cabochons. These bands are often in concentric or zigzag patterns of alternating white and pink (or red) colors. The transparent red forms large enough to be cut into gemstones are extremely valuable and cherished by rare gemstone connoisseurs.

Chemical Formula	MnCO3
Color	Red, Pink, Banded
Hardness	3.5Â -Â 4
Crystal System	Hexagonal
Refractive Index	1.60Â -Â 1.82
SG	3.0Â -Â 3.3
Transparency	Transparent to nearly opaque
Double Refraction	-0.22
Luster	Vitreous
Cleavage	1,3 - rhombohedral
Mineral Class	Rhodochrosite