Obsidian
Obsidian, also known as volcanic glass, is a rock rather than a mineral that is a mixture of cryptocrystalline grains of silica minerals in a glass-like suspension. It is produced from volcanoes when certain types of lava cool rapidly by flowing into a body of water. Obsidian consists mainly of silicon dioxide and is mineral-like, but not a true mineral because it has no crystal structure and therefore is amorphous. Otherwise, it is very similar in composition to granite and rhyolite. It is sometimes classified instead as a mineraloid. Most Obsidian is typically opaque black and not desirable for faceting.
There are several types of Obsidian typically offered as gems. The most common are opaque cabochons of black, Mahogany, Snowflake or Rainbow Obsidian. Mahogany Obsidian has brownish-red patches or swirls of black Obsidian while Snowflake Obsidian has greyish white patches of black Obsidian. Rainbow Obsidian has an iridescent, rainbow-like sheen. There are also translucent Obsidian gems with white inclusions of Cristobalite. These are often called Cosmic Obsidian.
Another type of Obsidian is called “Apache Tears” because of a legend of the Apache tribe. The legend is that about 75 Apache warriors and the US Cavalry fought on a mountain overlooking what is now Superior, Arizona in the 1870s. Facing defeat, the outnumbered Apache warriors rode their horses off the mountain to their deaths rather than be killed by the soldiers. The wives and families of the warriors cried when they heard of the tragedy and their tears turned to stone upon hitting the ground. Apache Tears are usually translucent to transparent and medium brown in color.
Green “Obsidian”
This mysterious gem has been called many names including Green Obsidian, African Moldavite, Tanzanian Tektite, Green Volcanic Glass and green man-made glass. One source says the Tanzanian natives call it Obsidianite. It is reported to be found in alluvial deposits and in a river bed at the base of Mt. Kilimanjaro. Some say it is a natural volcanic glass, some argue that it is man-made glass, others claim it is of meteoric origin similar to Moldavite. Where ever this material comes from or however it was formed, it is a stunning gem of the finest Tsavorite green color.
Please note: “Transparent blue or green Obsidian”
A lot of gem-quality water-clear brightly colored “Obsidian” has been offered for sale on the internet with a variety of sources listed. The material offered for sale is, in fact, a man-made glass produced in places such as Indonesia and China.
| Category: | Volcanic glass |
| Chemical Formula: | 70 – 75% SiO2 + MgO, Fe3O4 |
| Silicon Dioxide + Magnesium Oxide and/or Iron Oxide |
| Crystallography: | Amorphous, no crystal structure |
| Crystal Habit: | n/a |
| Twinning: | n/a |
| Cleavage: | Conchoidal |
| Fracture: | Conchoidal |
| Tenacity: | Brittle |
| Moh’s Hardness: | 5.5 – 6.0 |
| Density: | 2.33 – 3.00 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Not Radioactive |
| Color: | Black, bluish, brown, red, mahogany, golden, rainbow; the colors are largely due to refraction by microscopic bubbles and/or microscopic mineral inclusions. |
| Transparency: | Transparent to Translucent |
| Luster: | Vitreous (glassy) |
| Refractive Index: | 1.48 – 1.51 (Isotropic) |
| Birefringence: | 0.000 (Isotropic) |
| Dispersion: | n/a |
| Pleochroism: | n/a |
