coming from a mineralogy/gemology standpoint: corundum stones including ruby and sapphire are extremely heat resistant. Almost all natural sapphires are heat treated at high temperatures (1500C) to improve clarity and color. Heat treatment dissolve and disperses rutile (titanium dioxide) “silk” inclusions that cloud a stone… despite being heat resistant, these gems are fairly resistant but still susceptible to thermal shock. Corundum stones do not need to be removed from their settings when jewerly pieces are worked on, do not need a flux coating to prevent oxidation… cheaper stones, especially premade beads do have crack and strain inclusion making them susceptible to cracking with heat. Good quality cabochons are as heat resistant as cut stones… beryl stones, especially aquamarine, are also heat treated but the window of temperature is very narrow… overheating an acqua will discolor it, turn it yellow or green…same goes for other beryl stones except for emeralds… emeralds are inherently full of cracks… they cannot be heated. low quality rubies and sapphires can have cracks filled with glass, or resin… emeralds are most oftenly oiled… these stones cannot be heated safely, Diamonds in general have some heat resistance but the caveats were already listed… I never would consider exposing an expensive diamond to heat. I would rather remove it from its setting and do repairs or other modifications on the metal…corundum stones are another matter… most of the time, if not highly included, these stones can withstand heat…any expensive gemstone $5K or above should be handled with extreme caution… a jewler is responsible for an equivalent replacement stone if it’s damaged.
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