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Refractive index readings of faceted gems can range around α = 1.651, β = 1.668, and γ = 1.689, with a biaxial positive birefringence of 0.037–0.038. With decreasing magnesium and increasing iron concentration, the specific gravity, color darkness and refractive indices increase, and the β index shifts toward the γ index.
Prasiolite is a rare stone in nature; artificial prasiolite is produced from amethyst. [4] Most amethyst will turn yellow or orange when heated, producing heat-treated amethysts which are often marketed as citrine , but some amethyst will turn green when treated.
Jet is found in two forms, hard and soft. [5] Hard jet is the result of carbon compression and salt water; soft jet may be the result of carbon compression and fresh water. [5] Despite the name they both occupy the same area of the Mohs scale with the difference being that soft jet is more likely to crack when exposed to changes in temperature. [6]
Carnelian (also spelled cornelian) [3] is a brownish-red mineral commonly used as a semiprecious stone.Similar to carnelian is sard, which is generally harder and darker; the difference is not rigidly defined, and the two names are often used interchangeably.
Hessonite or "cinnamon stone" is a common variety of grossular with the general formula: Ca 3 Al 2 Si 3 O 12. The name comes from the Ancient Greek : ἣσσων (hēssōn), meaning inferior ; [ 5 ] an allusion to its lower hardness and lower density than most other garnet species varieties.
Lazurite is a pigment (opalescent) and has a bright blue streak (especially as a component of the semiprecious stone lapis lazuli). Many hauynes have a white or pale blue streak and are translucent. The difference might be a consequence of the redox state (sulfate to sulfide ratio). [8] [14]
The Jaredites placed a stone fore and aft on each ship and had "light continually" during their 344-day voyage to America (Ball 1938: 500). The theme of luminous gems guiding mariners and others originated in Europe in the Middle Ages. The earliest is probably the Scandinavian saga of the Visby garnets.
Opalescence or play of color is an optical phenomenon associated with the mineraloid gemstone opal, [1] a hydrated silicon dioxide. [2] This effect appears as a milky, translucent glow that changes with the angle of light, often creating a soft, pearly sheen that can display various colors or hues.