Loose gems for jewelry making Cabochons for jewelry-making in a variety of gemstones and glass. Black Onyx, Turquoise Matrix, Agate, Opalite, Tigereye. Gemstone Cabochons here include semi precious and glass cabochons. We sell loose flat-back cabochons and glass cabochons on the pages below. Narrow your search by clicking a link for any standard industry size.