An excellent obsidian rock specimen found in the United States of America, exhibiting in perfect detail the conchoidal fracture which this rock possesses. Obsidian is a black, glassy textured rock of volcanic origin, its texture is the result of lava rapidly cooling, such as if it comes in to contact with cold water.This rapid cooling does not allow the minerals within the rock to form slowly, instead instantly forming as the lava is cooled, which in turn results in very small, interlocking crystals which are so fine, the rock appears to look like black glass. Due to it's conchoidal fracture, which on a broken surface, appears as circular patterns, the rock fragments into sharp edged shards, which early humans have used in primitive stone age tools and weapons such as knives, daggers, arrow heads and more. This particular sample exhibits the fracture perfectly and so is particularly suited for an educational collection, where one can not only verbally explain this rock to students but visually show them as well. It measures 90 x 60 x 30 mm in size, care is advised when handling the sample, as it does have extremely sharp edges. The piece will come supplied in a card tray with an information label.