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2. Amethyst - Wikipedia

   en.wikipedia.org/wiki/Amethyst

   An amethyst geode that formed when large crystals grew in open spaces inside the rock. The largest amethyst geode found as of 2007 was the Empress of Uruguay, found in Artigas, Uruguay in 2007. It stands at a height of 3.27 meters, lies open along its length, and weighs 2.5 tons. Amethyst is also found and mined in South Korea. [19]

3. Geode - Wikipedia

   en.wikipedia.org/wiki/Geode

   Quartz-filled geode, shown from inside (top) and outside (bottom) A geode (/ ˈ dʒ iː. oʊ d /; from Ancient Greek γεώδης (geṓdēs) 'earthlike') is a geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which masses of mineral matter

4. Asterism (gemology) - Wikipedia

   en.wikipedia.org/wiki/Asterism_(gemology)

   Asterism is generated by reflections of light from twin-lamellae or from extremely fine needle-shaped acicular inclusions within the stone's crystal structure. [1] A common cause is oriented sub-microscopic crystals of rutile within the gem mineral. It occurs in rubies, sapphires, garnet, diopside, and spinel when a cabochon is cut from a ...

5. List of U.S. state minerals, rocks, stones and gemstones

   en.wikipedia.org/wiki/List_of_U.S._state...

   ^ In 2009, West Virginia named bituminous coal as its official state rock, in a resolution that noted that the coal industry plays an "integral part of the economic and social fabric of the state". West Virginia joined Kentucky and Utah , which also recognize coal as a state mineral or rock.

6. Porphyry - Wikipedia

   en.wikipedia.org/wiki/Porphyry

   Porphyry (geology), an igneous rock with large crystals in a fine-grained matrix, often purple, and prestigious Roman sculpture material; Shoksha porphyry, quartzite of purple color resembling true porphyry mined near the village of Shoksha, Karelia, Russia; Porphyritic, the general igneous texture of a rock with two distinct crystal ...

7. Kimberlite - Wikipedia

   en.wikipedia.org/wiki/Kimberlite

   Olivine crystals (green) are in a fine-grained matrix made up of clay minerals and carbonates (presented in blue, purple and buff colors). Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known to be the main host matrix for diamonds.