Topaz is a gemstone.
In the holodeck adventure Julian Bashir, Secret Agent, Bashir, posing as geologist Patrick Merriweather, had his knowledge of gemstones tested by Hippocrates Noah, showing him a statue encrusted with various kinds of gems, including topaz. Bashir successfully surmised that the statue contained not only topaz, but rubies, sapphires and tormaline, as well. (DS9: "Our Man Bashir")