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March 30, 2007, Newsletter Issue #133: Texture - Glassy
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Tip of the Week
Some igneous rocks have glassy texture. That doesnīt mean the rock shines like glass (although some might!). It means that the lava cooled so quickly that individual crystals didnīt form at all. In other words, there are no individual crystals, even under a microscope. Glassy igneous rocks often seem to be a solid color, like light gray or black.
Lots of folks get "glassy" confused with "shiny." Obsidian has a glassy texture, and even looks shiny, like glass. But pumice, which also has glassy texture, is a dull rock which doesnīt shine at all.
How do you know if a rock is fine-grained (has individual mineral grains in it, but you canīt see them without a hand lens or even a microscope) or glassy (no individual crystals at all)? You canīt tell, just by looking. But obsidian, pumice, scoria, and tuffs have a glassy texture. Other igneous rocks that seem to be more-or-less a solid color, like basalt, rhyolite, and andesite, have an aphanitic (fine-grained) texture.
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