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
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
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
(By Dolores E. Rose, from Stoney Statements, 4/2001)