Obsidian Crystal is an igneous rock and a type of natural glass. It forms when molten lava cools rapidly, resulting in the congealing of minerals within it into a solidified mass. Its mineral group consists mainly of silicon dioxide, which gives this stone its trademark black color. The conversion of loose sediment into solid sedimentary rock. Several processes, including compaction of grains, filling of spaces between grains with mineral cement, and crystallization act to solidify sediment.