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Obsidian talus at Obsidian Dome, California Polished snowflake obsidian, formed through the inclusion of cristobalite crystals. The Natural History by the Roman writer Pliny the Elder includes a few sentences about a volcanic glass called obsidian (lapis obsidianus), discovered in Ethiopia by Obsidius, a Roman explorer. [9] [10] [11] [12]
These rocks are usually found within obsidian or rhyolite lava flows. [2] Lavas low in feldspar minerals may produce a version known as snowflake obsidian . These cavities are believed to be caused by expanding gases in tuffs and rhyolitic lavas before solidification. [ 1 ]
Opals too thin to produce a "natural" opal may be combined with other materials to form "composite" gems. An opal doublet consists of a relatively thin layer of precious opal, backed by a layer of dark-colored material, most commonly ironstone, dark or black common opal (potch), onyx, or obsidian. The darker backing emphasizes the play of color ...
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Most igneous rocks have some degree of porphyritic texture. This is because most magma from which igneous rock solidifies is produced by partial melting of a mixture of different minerals. [4]
Spherulites in rhyolitic ash, Hailstone Trail, Echo Canyon, Chiricahua Mountains, Arizona Spherulite markings on snowflake obsidian Photomicrograph of rhyolite, showing spherulitic texture (brown, between grey to white crystals) In petrology, spherulites (/ ˈ s f ɛr ʊ l aɪ t s, s f ɪər-/) are small, rounded bodies that commonly occur in ...