As a scientist, I will give you the facts. Opal is a relatively soft gemstone. Diamond is rated 10 on the Mohs scale, while opal is between 5 and 6. Opal is comparable to modern glass. Opals are rarer than diamonds. While diamonds are set as studded solitaires, most opals are set with a gold rim around the outer edge to help protect the stone. While diamonds display all the colors of the rainbow just like a refracted prism, opal actually has all the colors of the rainbow in the gem itself. When one looks at an opal stone, the moment of color is in an unpredictable pattern and is referred to as a play of colors.