Obsidian is a natural volcanic glass, an extrusive igneous rock, it is produced when silica rich lava extruded from a volcano rapidly cools minimising crystal growth which gives the rock a fine, glassy texture. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows, its high silica content (70 -75%) produces a highly viscous lava with some degree of polymerization. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. It is hard and brittle rock with a black appearance due to the presence of conchoidal fractures, it therefore breaks with very sharp edges. It is this particular feature which has made obsidian an important rock in human history and evolution and why it is so well known and popular, as the sharp edges allowed it to be used in the past in cutting and piercing tools, most notably, arrow and spear heads. It has been used experimentally as scalpel blades in the past and recent years. These particular samples come from South East Mexico, the pieces will display typical black colouration and conchoidal fractures, making them suitable for both collectors, enthusiasts and educational use. Each sample will come supplied in a card tray with an information label.
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