Cristobalite
Cristobalite is really a polymorph of all of the of this understood members for the Quartz Group of minerals. These include Quartz, Coesite, Tridymite and Stishovite. A polymorph means these minerals have the chemistry that is the same SiO2, but different structures. Cristobalite is common as microscopic crystals in volcanic stones but larger, well-formed crystals found in the vesicles of volcanic rocks are rare. Cristobalite itself could not be faceted, but bright cristobalite that is white captured in transparent black colored Obsidian (as pictured above) make very attractive gems. These gems have been called Cosmic Obsidian. Material from Lipari Island, Messina Province, Sicily, Italy is the source that is mainly of the type of gem.
Notable occurrences include the kind locality of Cerro San Cristóbal, Mun. de Pachuca, Hidalgo, Mexico; Eifel District, Germany; the Ellora Caves, Maharashtra, India; Lipari Island, Messina Province, Sicily, Italy; Goroyama, Japan; about 150 north that is kilometer of, New Zealand; in America in the San Juan Mountains, San Juan County, Colorado; Coso Hot Springs, Inyo County, California; near Crater Lake, Klamath County, Oregon; and many other places.
| Category: | Oxide mineral, quartz group |
| Chemical Formula: | SiO2 |
| Silicon Dioxide | |
| Molecular Weight: | 60.08 gm |
| Composition: | Silicon | 46.74 % | Si | 100.00 % | SiO2 |
| Oxygen | 53.26 % | O | |||
| 100.00 % | 100.00 % | = TOTAL OXIDE |
| Crystallography: | Tetragonal – Trapezohedral |
| Crystal Habit: | As pseudo-octahedral crystals, to 4 mm, with {110} and {331}, rarely pseudocubic. Commonly dendritic to skeletal; as spherulites to several cm; fibrous or microcrystalline (“opal”), massive. |
| Twinning: | On {111}, common, interpenetrant, polysynthetic, repeated. |
| Cleavage: | None |
| Fracture: | Irregular/Uneven |
| Tenacity: | Brittle |
| Moh’s Hardness: | 6.0 – 7.0 |
| Density: | 2.32 – 2.36 (g/cm3) |
| Luminescence: | Fluorescent; LW UV = Green Red; LW UV = Green Red |
| Radioactivity: | Not Radioactive |
| Other: | Thermal properties: inverts from high- or ß-Cristobalite at 286°C or below |
| Color: | Colorless, White, milky White to Yellowish, Blue-Gray, Gray; Colorless in transmitted light. |
| Transparency: | Transparent to Translucent |
| Luster: | Vitreous |
| Refractive Index: | 1.484 – 1.487 Uniaxial ( – ) |
| Birefringence: | 0.003 |
| Dispersion: | None |
| Pleochroism: | None |
