“Topazius” is one ancient name for the stone we know as peridot (the mineral olivine, in gem-quality form.) Another name for it is “chrysolite” (no longer really used, as it could be confused with chrysoberyl.) The ancient texts on gemstones make it challenging, as one name can mean several different gem species (sapphire = anything blue, etc.) To make matters worse, an entirely different stone has almost the same name (derived from a different word.)
The truth is,
chrysolite.com was taken, and “topazius” made such a cute .us domain although I’ve always liked “chrysolite”, there’s a neat sound to “topazius”, and I like that it represents the ancient source of one of my favorite gemstones. (And it does make a cute .us domain. It’s fun!)
This gem derived its name from the island in the Red Sea, 300 stadia (30 miles) off the mainland, where it was first discovered …
… The gem was not discernible by day, its lustre being then overpowered by the sun’s rays, but at night was conspicuous by its brightness ; the guards, who divided the island among their patrols, then ran up and covered the luminous spot with a vase of equal size. Next day they go their rounds, and cut out the patch of rock thus indicated, and deliver it to the proper persons to be polished.
This stone was indubitably our Chrysolite, or Peridot ; the distinctive characters of which exactly agree with those pointed out by Pliny. His Topazius was imported from some place in the Red Sea … it was of a bright yellowish-green, a colour peculiar to itself (in suo virenti genere), and was the softest of all the precious stones, yielding to the file, and suffering from wear.
— The Natural History of Precious Stones and Gems, by C. W. King
See also a recent article in Minerva Magazine on this ancient source of peridot, courtesy of Pala Gems.