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Your specimen is probably obsidian. Obsidian forms when a
volcano with dense, viscous lava erupts and cools very quickly. This
rapid cooling produces incredibly fine crystals which give the rock a glassy
texture. Broken obsidian usually has a conchoidal fracture (as you would
see in a thick piece of broken glass).
Obsidian is usually dark brown, red or black but can sometimes have white
patches. Specimens with these patches are known as snowflake obsidian.
A number of igneous rocks from The Potteries Museum &
Art Gallery collections are featured on the Virtual
Store.
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