A ring stone is a type of jewelry that is set into a ring. The four solutions to the crossword are ONYX, GEM, OPAL, and CAMEO. ONYX is a black or dark-colored gemstone that is often used in jewelry. A GEM is a precious or semi-precious stone that is cut and polished for use in jewelry. OPAL is a gemstone that has a milky iridescence and can be found in a variety of colors. Finally, a CAMEO is a gemstone that is carved with a raised relief image.