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Crystal Formation & Sedimentary Rock

The Rock Cycle   |  Igneous Rock   |  Metamorphic Rock   |  Sedimentary Rock

Types of 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

Fossilization & Petrification

Fossilization is a process where an a living organism's remains are entombed in sediment, preserving the original hard parts (bone, shell, or wood); or creating a cavity or impression of soft parts (flesh, leaves, etc.); or creating a mineralized version of the hard remains through a process called "replacement" or "permineralization." Permineralization occurs when the original hard parts are buried within sediment, then ground water carrying dissolved minerals will infiltrate the microscopic pores of these parts, depositing their mineral content within.

Petrification typically combines the process of permineralization and replacement, with the original hard parts being buried within the sediment, then dissolved by chemical action, and slowly replaced by minerals such as silica and pyrite. With permineralization the original structure is preserved, whereas with replacement the original structure is imperfectly preserved.

After petrification, the material takes on the physical properties of a rock-like cryptocrystalline mineral. Petrification can also occur when an organism comes to rest in an anoxic (oxygen-free) environment such as at the bottom of a lake or on the sea-bed.

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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 .

3. Merguerian, Geologic Structure - A Primer .

4. D. Nahon, Y. Wang, Pseudomorphic Replacement .

5. Walter Schumann, Gemstones of the World . NAG Press; 2Rev Ed edition

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