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the four most precious types of gems, the other three being diamond , emerald , and sapphire . in chemical and mineralogical terms, the ruby is a variety of the mineral corundum (aluminum oxide), with the red color produced mainly by the presence of small amounts of chromium . the non-red varieties
sapphires) can be found in various rocks, mainly those of igneous [ ] or metamorphic [ ] origin (sethi, ). in addition, through the weathering of gem-bearing sedimentary rocks, these minerals are found in the form of "placer deposits" at streams and rivers. for instance, in the mogok region of myanmar...
https://www.newworldencyclopedia.org/entry/Ruby