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2. Gemstone irradiation - Wikipedia

   en.wikipedia.org/wiki/Gemstone_irradiation

   Pearls are irradiated to produce gray blue or gray-to-black colors. [22] Methods of using a cobalt-60 gamma ray facility to darken white Akoya pearls were patented in the early-1960s. [29] But the gamma ray treatment does not alter the color of the pearl's nacre, therefore is not effective if the pearl has a thick or non-transparent nacre. [29]

3. Shades of gray - Wikipedia

   en.wikipedia.org/wiki/Shades_of_gray

   An achromatic gray is a gray color in which the red, green, and blue codes are exactly equal. The web colors gray, gainsboro, light gray, dark gray, and dim gray are all achromatic colors. A chromatic gray is a gray color in which the red, green, and blue codes are not exactly equal, but are close to each other, which is what makes it a shade ...

4. Chalcedony - Wikipedia

   en.wikipedia.org/wiki/Chalcedony

   Chalcedony. Chalcedony (/ kælˈsɛdəni / kal-SED-ə-nee, or / ˈkælsəˌdoʊni / KAL-sə-doh-nee) [2] is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. [3] These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic.

5. Smoky quartz - Wikipedia

   en.wikipedia.org/wiki/Smoky_quartz

   Smoky quartz. Smoky quartz is a brownish grey, translucent variety of quartz that ranges in clarity from almost complete transparency to an almost-opaque brownish-gray or black crystals. [6] The color of smoky quartz is produced when natural radiation, emitted from the surrounding rock, activates color centers around aluminum impurities within ...

6. List of RAL colours - Wikipedia

   en.wikipedia.org/wiki/List_of_RAL_colours

   Possible lightness values are 15% through 90% in steps of 5% for monochromatic shades of grey (i.e. C = 0%) and 20% through 90% in steps of 10% and additionally 85% and 93%. Chroma values are also mostly confined to steps of 10% or 5%, but with some exceptions, and the maximum value varies.

7. Quartz - Wikipedia

   en.wikipedia.org/wiki/Quartz

   Quartz. Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO 4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO 2. Quartz is, therefore, classified structurally as a framework silicate mineral ...

8. Larvikite - Wikipedia

   en.wikipedia.org/wiki/Larvikite

   Larvikite. Polished larvikite (marketed as "Blue Pearl Granite"), showing labradorescence, is a popular decorative stone. Larvikite is an igneous rock, specifically a variety of monzonite, [1] notable for the presence of thumbnail-sized crystals of feldspar. These feldspars are known as ternary because they contain significant components of all ...

9. Greywacke - Wikipedia

   en.wikipedia.org/wiki/Greywacke

   Greywacke or graywacke (German grauwacke, signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size lithic fragments set in a compact, clay -fine matrix.