Iron and magnesium typically give the obsidian a dark green to brown to black color. While pure obsidian is always dark in appearance, the color varies depending on the presence of impurities. Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now agree with this theory. The breakdown of obsidian is accelerated by the presence of water. It is sometimes classified as a Mineraloid because obsidian is metastable at the earth's surface (over time the glass becomes fine-grained mineral crystals). Its composition is very similar to that of granite and rhyolite. Obsidian is mineral-like, but not a true mineral because it is not crystalline. While a rock like basalt is dark because of ferromagnesian enrichment, obsidian consists mainly of silicon dioxide, usually 70% or more. Because of the lack of crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as arrowheads and its modern use as surgical scalpel blades. They are extremely sharp, even sharper than surgical steel scalpels. It is commonly found within the margins of felsic lava flows, where cooling is more rapid. The obsidian rock will show a unique flaking pattern that tapers to the blade.
an ideal material to make surgical scalpels that have cutting edges that are. Obsidian is a rock which is a type of naturally occurring glass, produced by volcanoes (igneous origin) when a felsic lava cools rapidly and freezes without sufficient time for crystal growth. Obsidian is quite unique due to its smooth, uniform volcanic glass texture.
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