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  2. Nuummite - Wikipedia

    en.wikipedia.org/wiki/Nuummite

    In Greenland the rock was formed by two consecutive metamorphic overprints of an originally igneous rock. [3] The intrusion took place in the Archean around 2800 million years ago and the metamorphic overprint was dated at 2700 and 2500 million years ago.

  3. Tanzanite - Wikipedia

    en.wikipedia.org/wiki/Tanzanite

    Clarity grading in coloured gemstones is based on the eye-clean standard, that is, a gem is considered flawless if no inclusions are visible with the unaided eye (assuming 20/20 vision). [29] The Gemological Institute of America classifies tanzanite as a Type I gemstone, meaning it is normally eye-clean.

  4. Metamorphic zone - Wikipedia

    en.wikipedia.org/wiki/Metamorphic_zone

    In geology, a metamorphic zone is an area where, as a result of metamorphism, the same combination of minerals occur in the bedrock. These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and pressure .

  5. Tiger's eye - Wikipedia

    en.wikipedia.org/wiki/Tiger's_eye

    Tiger's eye (also called tiger eye) is a chatoyant gemstone that is usually a metamorphic rock with a golden to red-brown colour and a silky lustre.As members of the quartz group, tiger's eye and the related blue-coloured mineral hawk's eye gain their silky, lustrous appearance from the parallel intergrowth of quartz crystals and altered amphibole fibres that have mostly turned into limonite.

  6. Metasomatism - Wikipedia

    en.wikipedia.org/wiki/Metasomatism

    In the igneous environment, metasomatism produces skarns, greisen, and may affect hornfels in the contact metamorphic aureole adjacent to an intrusive rock mass. In the metamorphic environment, metasomatism is driven by mass transfer from a volume of metamorphic rock at higher stress and temperature into a zone with lower stress and temperature ...

  7. Beryl - Wikipedia

    en.wikipedia.org/wiki/Beryl

    While gem beryls are ordinarily found in pegmatites and certain metamorphic stones, red beryl occurs in topaz-bearing rhyolites. [53] It is formed by crystallizing under low pressure and high temperature from a pneumatolytic phase along fractures or within near-surface miarolitic cavities of the rhyolite.

  8. Greenschist - Wikipedia

    en.wikipedia.org/wiki/Greenschist

    Chlorite schist, a type of greenschist Greenschist (prasinite) at Cap Corse in Corsica, France Greenschist (epidote) from Itogon, Benguet, Philippines. Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically 300–450 °C (570–840 °F) and 2–10 kilobars (29,000–145,000 psi). [1]

  9. Index mineral - Wikipedia

    en.wikipedia.org/wiki/Index_mineral

    Map of index minerals of the Barrovian metamorphic zones of Scotland. An index mineral is used in geology to determine the degree of metamorphism a rock has experienced. . Depending on the original composition of and the pressure and temperature experienced by the protolith (parent rock), chemical reactions between minerals in the solid state produce new mi

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