Round brilliant-cut cassiterite, 1.62 cts, 5.5 mm, dark yellowish brown, Bolivia. © The Gem Trader. Used with permission.
Cassiterite is a durable gemstone with tremendous dispersive fire, especially visible in properly cut pale-colored stones. As the principal ore of tin, it’s also a common mineral. Unfortunately, facetable rough is very scarce.
| Data | Value |
|---|---|
| Name | Cassiterite |
| Varieties | Wood Tin |
| Crystallography | Tetragonal. Crystals prismatic, pyramidal; also botryoidal, reniform with a radial fibrous structure. Twinning common. |
| Crystallographic Forms | |
| Refractive Index | 2.006-2.101 |
| Colors | Brown, brownish black, black, colorless, gray, yellowish, greenish, red. |
| Luster | Adamantine to vitreous; greasy on fracture surfaces. |
| Hardness | 6 - 7 |
| Fracture | Subconchoidal to uneven |
| Specific Gravity | 6.7–7.1; pure material 6.99 |
| Birefringence | 0.098 |
| Cleavage | Imperfect |
| Dispersion | 0.071 (nearly twice that of diamond) |
| Luminescence | None. |
| Luminescence Present | No |
| Transparency | Transparent to opaque. |
| Absorption Spectrum | Not diagnostic. |
| Phenomena | Chatoyancy. Some cabochons may show artificial asterism. See “Enhancements” below. |
| Formula | SnO2 + Fe, Ta, Nb. |
| Pleochroism | Weak to strong; greenish yellow or yellow-brown/red-brown. Most visible in strongly colored crystals. |
| Optics | o = 2.006; e = 2.097 – 2.101. Uniaxial (+); anomalously biaxial, 2V = 0-38°, usually in zoned crystals. |
| Optic Sign | Uniaxial + |
| Etymology | From the Greek kassiteros for “tin,” due to its composition. |
| Occurrence | Principal ore of tin; occurs in medium to high temperature veins; metasomatic deposits; granite pegmatites; rhyolites; alluvial deposits. |
| Inclusions | Usually veils, two-phase inclusions. Some Bolivian material may contain tourmaline needles. |
Twinned, lustrous black cassiterites, each crystal approximately 3.0 cm in length, Schlaggenwald, Erzgebirge, Bohemia, Czech Republic. © Rob Lavinsky, www.iRocks.com. Used with permission.
Most cassiterite crystals have opaque, dark colors, usually black or brown. Lighter-colored, facetable crystals are much rarer and usually occur in small fragments. With a dispersion or fire almost twice that of diamond as well as vitreous to adamantine luster, transparency, and a hardness of 6-7, faceted cassiterites would make good jewelry stones. However, the combination of rarity and little consumer awareness means you’ll more likely find these for sale as collector’s specimens.
Cassiterite rough and cut set. Cushion-cut gem: 13.5 cts, Sri Lanka; crystal: 2.1 x 1.8 x 1.5 cm, Viloco Mine, La Paz, Bolivia. © Rob Lavinsky, www.iRocks.com. Used with permission.
Cassiterites belong to the rutile mineral group, along with rutiles, of course, as well as other non-gem mineral species.
Not surprisingly, as the world’s main tin ore, cassiterite has been called “tin stone” and “tin spar.” Some varieties also have names associated with tin. “Dough tin,” a white-colored variety from Cornwall, England, has a texture like unbaked bread dough. “Toad’s eye tin” looks like dough tin but has botryoidal (sphere or grape-like) or reniform (kidney-like) crystal structures. “Stream tin” occurs in round, water-worn shapes.
“Stream tin” cassiterite, 4.0 x 3.3 x 1.5 cm, Basin Creek placers, Cataract District (Comet District; Basin District), Jefferson Co., Montana, USA. © Rob Lavinsky, www.iRocks.com. Used with permission.
“Wood tin,” a cryptocrystalline variety of cassiterite, has bands that resemble the age rings of wood. This variety occurs in locations across the globe, and lapidaries sometimes create ornamental objects and cabochons from it.
“Wood tin” cassiterite, La Paz Department, Bolivia. Photo by James St. John. Licensed under CC By 2.0.
Cassiterite has “over the limit” (OTL) refractive indices of 2.006-2.101.
Faceted transparent cassiterites may show birefringent effects such as doubling and fuzziness.
The most distinguishing property of this gemstone is its very high specific gravity (SG) range of 6.7-7.1. All gems of this density or higher are rarely faceted, but cassiterite probably is the most likely to be cut. Other highly dispersive, very dense, rarely faceted gems include stibiotantalite and cinnabar. However, both exceed cassiterite’s SG and have lower hardness.
Sphalerites may resemble cassiterites in color range and luster. However, they have an even higher dispersion than cassiterites and significantly lower SG and hardness values.
Cut black cassiterites may be mistaken for natural black diamonds due to their adamantine luster. However, cassiterites have a far greater SG than diamond. While cassiterites make fairly durable gemstones, their hardness of 6-7 falls well below diamond’s 10.
Cassiterites may also display color zoning.
Rectangular step-cut cassiterite, 6.4 cts, 10.8 x 6.6 mm, with medium and dark brown color zoning, Bolivia. © The Gem Trader. Used with permission.
As an important commercial source of tin, cassiterite crystals have been synthesized via the hydrothermal method for research into their properties as well as their geological development. Since cassiterites have desirable gemstone qualities, synthetics could find a place in jewelry, whether as synthetic cassiterites or as simulants of more well-known gems. However, to date, there are no known appearances of such synthetics in jewelry.
A 2001 GIA investigation into displays of asterism in gems not known to show this effect revealed a new way to artificially “asteriate” gems. The gems tested included a supposed “star cassiterite.” Evidently, tightly packed, oriented, coarse lines were scratched parallel to each other manually, likely with a polishing wheel, on the domed surfaces of cabbed stones. Although some specimens looked very convincing, most appeared unnatural, with asymmetrical rays in unusual numbers. The star effects also lacked depth.
While polishing errors can also inadvertently create a star-like effect, these gems appear to have been deliberately scratched to create this appearance. Asterism can drive up the price of a stone, therefore, buyer beware.
While the mineral cassiterite occurs in many locations across the globe, Bolivia produces most of the gem material known. The Araca Mine yields yellow, gray, colorless and light yellowish brown to reddish brown stones.
China and Russia also produce gem-quality cassiterites.
Other notable gem sources include the following:
Yellow, round-cut cassiterite, 2.80 cts, 6.7 x 4.7 mm, China. © ARK Rare Gems. Used with permission.
Clean cassiterites very rarely weigh over 1 carat. Opaque masses occur up to several pounds in weight. Lapidaries sometimes cut them into cabochons. Pale brown to dark brown gems up to 15 carats have been cut, while slightly flawed stones up to 25 carats are also known. Gems of this size are mostly Bolivian material.
Although cassiterites have an imperfect cleavage plane, this doesn’t pose a major problem for wear. Keep in mind that at a hardness of 7, a cassiterite matches quartz in durability and is suitable for any jewelry use and everyday wear. At 6, however, exercise more caution when setting and wearing your cassiterite.
Most cassiterites contain inclusions, so it would be best to have a gemologist determine if your gem is suitable for mechanical cleaning. Otherwise, a soft brush, mild detergent, and warm water makes a safe alternative. Consult our gemstone jewelry cleaning guide for more recommendations.
Cassiterite: Bolivia (14.25, 3.5). Photo © Joel E. Arem, PhD, FGA. Used with permission.
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