#volcanic_glass  a kind of natural glass produced when molten lava cools very rapidly
  supertype:  volcanic_rock  extrusive igneous rock solidified near or on the surface of the Earth
  subtype:  obsidian  acid or granitic glass; usually dark, but transparent in thin pieces
  subtype:  pitchstone  dark acid granitic glass
  subtype:  tachylite  a basic or basalt glass

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