The word here for “red gemstone” is “anthrax”. This could be translated as “coal”. Coal would certainly be correct according to Strong's, however the Greek-English ancient lexicon that tracks the use of the words in ancient times indicates that in ancient writings, the word "anthrax" was also a reference to "a precious stone of dark-red color, including the carbuncle, ruby, and garnet". Indeed, this is the usage seen in the 4th century BCE in Aristotle's writings and those of the Greek historian Phylarchus.
The same word already appears in our translation at Exodus 28:18 as "carbuncle", which is an archaic term for a red ruby, as it was one of the precious stones to be put in the High Priest's breast plate.
It can certainly also mean a burning hot coal, though. It is used in Isaiah 6:6 in the Greek Septuagint to describe the hot coal taken from the altar by the angel. So the mystery may be solved by thinking of anthrax as meaning any red-colored mineral, whether it's red because it's just naturally that color, or red because it's very hot. If that's true, then the reference to it in Genesis 2:12 would not be to coal.
The Hebrew version of Genesis 2:12 calls it "bedolach" (or, "bdellium"), which was thought to be amber or some other yellow-like stone.
Also, the context of Genesis is, of course, listing precious stones and metals. So coal would not fit the context either, although admittedly, it is a very short list, so perhaps this is not a valid objection.
So the word here could mean "amber", "ruby", "precious red stone", or "red gemstone". Of course, amber is not red, nor is bdellium, but most ancient languages had very few color names, and made no distinction between red, orange, and yellow; they were all just called "red".
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