Obsidian is silica, atoms of silicon and oxygen bonded together to form silica molecules. If liquid silica cools very slowly, it becomes quartz crystals. If the cooling is relatively fast, the solid result is a silica glass, the stuff, when synthesized, from which most windows are made. Obsidian is a glass of volcanic origin.
A volcano is a crack in the earth’s crust through which molten material is thrown out of the deeper part of the earth onto the surface, where is cools quite rapidly. Molten silica expelled from deep in the earth in this way forms the silica glass called obsidian.
To understand better what this really is, here is a short explanation of how some mineral crystals form.
Many crystals, including all forms of quartz and feldspar, start their life as a liquid at very high temperature and under great pressure deep in the earth. As they rise to the surface, pressure reduces and the liquid cools. Sometimes this cooling is quite fast, but mostly it is very slow.
When the cooling is slow, the atoms that make up the liquid material have time to settle into very regular and repeating patterns. As the mass solidifies, the atoms are no longer able to move as they could in a liquid state. When the atoms are arranged in a single, regularly repeating pattern throughout the solid, a crystal is formed. Single crystals may be so small as to be invisible under an optical microscope, or they can be meters long.
When the cooling happens quickly, the atoms are fixed in permanent positions before they can settle into one regular and repeating pattern. This solid state is called a glass. It’s the same stuff, chemically, but has different physical characteristics.
As a natural glass, obsidian is never pure and will contain sometimes large amounts of all sorts of other material and bubbles too, some of interesting shapes. Like flint (a form of chalcedony), obsidian (and any other silica glass) can be chipped carefully to produce a sharp cutting edge. These two forms of silica, one crystalline and one not, became the first edged tools--knives, axes, spear points and, later, arrowheads--that prehistoric man learned to make.
Obsidian is found in black, brown, green (rare) and gray and it has been used since antiquity for adornment. In modern times it is easily faked from synthetic silica glass of modern production. There is no certain test for differentiating the real from the fake but only good experience of types and shape of other materials and the bubbles commonly found in obsidian.
Obsidian will usually occur wherever there is or has been volcanic activity. Important sources are Mexico, Ecuador, Iceland, Italy and Indonesia.
Snowflake obsidian image courtesy of gemstoneslist.com.
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