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rock > aphanitic rock > obsidian

obsidian
subject			fact

obsidian	is a kind of volcanic rock
	is a kind of holohyaline rock
	is a kind of aphanitic rock
volcanic rock	has solidification mechanism extrusion onto the surface of the planet or asteroid via a volcanic eruption
	has solidification timescale very rapid
holohyaline rock	has texture glass
	has crystalinity 0 %
aphanitic rock	has grain size too small to see with unaided eye or low power lens
igneous rock	has composition silicates (with very few exceptions)
rock	has part mineral
	has genesis