obsidian - teaching display/hand specimen of black obsidian
Obsidian is a volcanic glass with the composition of granite or rhyolite. Because it cooled quickly on the surface the atoms did not have enough time to fall into a regular crystalline pattern and are essentially an organized mass of atoms exhibiting the curved or conchoidal fracture typical of glass.
The color of obsidian is due to impurities. Black obsidian results when the impurities are hornblende, magnetite, biotite or a pyroxine. Brown obsidian forms when the impurity is hematite or limonite. Inclusions of gas or tiny inclusions of water result in a silvery reflectance that is desirable in lapidary. Brown and black are commonly swirled together in some obsidian flows.
Almost every obsidian source in North America was known to native Americans. Since obsidian from each source is geochemically distinct, the origin of the obsidian used in stone tools can be traced. Many surprisingly obscure sources, such as obsidian float in alluvium where the original flow has weathered away, were utilized for cutting tools or was collected as trading material.
These specimens can have sharp edges. If you are considering using one in a classroom, let us know and we can dull the edges by rubbing against a geologist's hammer. If destined for a private collection, be wary of the edges, dull them yourself, or have us do it.
Select a specimen: You can select a specimen by telling us what is in the photo with it, a blue or black and silver pen, a black mechanical pencil or one of those plus some number of coins, or you can let us make the selection.
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