Crystal Formation & Sedimentary Rock
Sedimentary Rock
Sedimentary rock is formed by minerals being transported from their origination point, then deposited (deposition) in a final resting place where they are eventually compacted into a heterogeneous conglomerate. The mixture of sediments can contain clay, organic material, gravel, silt, and sand, along with exotic trace elements.
Over the coarse of thousands of years, successive layers of sediment are compressed and compacted, with each layer containing varying amounts of different materials, thus creating a 'banded' effect.
Sedimentary rock can change into igneous rock when it is carried underground (deep burial) via subduction, thereby melting and/or vaporizing it to combine with mantel rock. Sedimentary rock can also change into metamorphic rock, when it is compressed and heated by seismic activity or deep burial (folding).
Basic types of sedimentary rock are gypsum (sulfate from evaporating sea water), limestone (calcite from sea shells and sea beds), sandstone (feldspar, quartz), shale (clay), and conglomerates (sand and pebbles, bonded with dissolved minerals).
Regions of Gemstone-Bearing Sedimentary Rock
Many gem varieties that are buried within sedimentary rock were transported from their primary location, then deposited in the form of alluvial pebbles that came to rest along ancient riverbeds.
Austrailia is a region associated with the formation of opal, which crystalizes within sedimentary layers as a result of compaction.
Gemstones Associated with Sedimentary Rock
Bibliography and Reference on Gem & Rock Formation
1. Paul R. Shaffer, Herbert S. Zim, Raymond Perlman, Rocks, Gems and Minerals  . Martin's Press
2. UC Berkeley, Pegmatites . ist-socrates.berkeley.edu
3. Merguerian, Geologic Structure - A Primer . people.hofstra.edu
4. Walter Schumann, Gemstones of the World . NAG Press; 2Rev Ed edition
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