Oddities of Obsidian

Obsidian is an extrusive igneous rock formed when the magma of an erupting volcano reaches the earth’s surface and cools rapidly. It is an extrusive rock because it was pushed out onto the surface. The cooling of the extrusive rock occurs so rapidly that the magma doesn’t form minerals at all, but a volcanic glass.

It derives its name according to Pliny, an ancient Roman naturalist, from a fellow named Obsius, who found it in Ethiopia. Originally, it was named “obsianus”, but the spelling was changed over the centuries to its modern form.

Obsidian occurs in many colors, black being the most common. It can also be red, brown or even green. It can contain inclusions of magnetite, ilmenite, iron oxide, potassium oxide, sodium, oxide, lime and magnesium. It is composed of 66-77% silica, with about 13-18% alumina. Magnetite most likely gives obsidian its black color, and oxidized magnetite or hematite the reds and browns.

With slow cooling, silica crystals Cristobalite form, giving the “snowflake” obsidian or “flowering” obsidian. Iridescence reflected from minute inclusions arranged in layers is known as “rainbow obsidian”. Another kind with gold inclusions with a strong metallic luster is called “gold sheen obsidian”, and if the inclusions are grayish silver in color, it’s called “silver sheen”.

Obsidian is interesting in many ways, but mainly, for all practical purposes, it is a true glass. It has a hardness of 5-5.5 on the Mohs hardness scale. It represents a quickly congealed mass of molten rock, for if it had time to cool slowly, it would have crystallized into a rock similar to granite or rhyolite.
It shows no trace of crystalline structure nor possesses any established composition and must be considered a rock instead of a mineral. It is amorphous, having no regular internal arrangement of atoms as in crystals. The word amorphous is taken from the Greek and means “no form” because there is no pattern to amorphous materials. The atoms are jumbled together in small groups like particles in a pile of sand. It is extremely brittle and breaks easily with shiny, black conchoidal fractures – a feature so perfectly developed that it is easily identifiable in the field. It is translucent and will not soften when heated to a bright red.

Obsidian is found throughout the western United States, mostly in Alaska, Colorado, Utah, New Mexico, Arizona, Wyoming, Oregon, Nevada and California. It is also found in B. C. and throughout Mexico. American Indians valued obsidian highly. Its perfect texture and easy fracture made it a prize possession for chipping into arrowheads and large ceremonial spear points.

The Aztecs called obsidian “iztli”, “teotal” or “divine stone” because of its usefulness in carving ceremonial blades. Even one of their gods was named “Itzoppziotl”, meaning “obsidian butterfly”.
Obsidian is also used to make attractive jewelry as cabochons or faceted. Thin slabs can be cut with a common glass cutter. Due to its extreme heat sensitivity, great care must be taken in working obsidian. Industries use obsidian as a raw material to make rock wool. Surgeons have even used thinly chipped obsidian knives in surgery because of the fine exact cut an obsidian knife makes.
(By Dolores E. Rose, from Stoney Statements, 4/2001)