Obsidian is a dark, glassy-looking igneous rock. Most obsidian contains
the same chemical elements as granite and rhyolite, since all three of
these rocks can form from the same type of molten rock material.
Obsidian, however, has no separate minerals, because its chemical
elements are not combined in an orderly way. It is a natural glass.
Because it is a glass, we know that obsidian forms very quickly. One way
for it to form is from the sudden cooling of hot, molten lava that flows
out of volcanoes. If the lava cools and hardens before the separate
minerals can crystallize, it becomes a natural glass, such as obsidian.
This rock is smooth and shiny. Most of it is black, but some can be dark
green or dark brown. Obsidian allows light to pass through it, and it
breaks with a curved, conchoidal fracture. The broken edges are very
sharp.
Another glassy igneous rock that forms from fast-cooling lava is
_vitrophyre_. It looks like obsidian except that it has crystals or
crystalline mineral grains (which may be light colored) scattered
through the dark glassy material.
[Illustration: Obsidian was used by the Indians to make arrowheads.]
Obsidian and vitrophyre are found in the Big Bend area of Brewster and
Presidio counties in west Texas. They occur with other igneous rocks
that formed there during Tertiary time.
The Indians who long ago roamed this area used the smooth, shiny
vitrophyre and obsidian to make some of their arrowheads and scrapers.
Today, rock collectors pick up these attractive rocks for their
collections, and some of them cut and polish obsidian and vitrophyre for
use as gemstones.
Onyx. _See_ Quartz.
Öolitic Limestone. _See_ Limestone.