The sapphire doublet, for example is blue glass with a natural garnet top. The harder exposed surface of the garnet protects the softer glass from rapid wear. Also common today are doublets consisting of natural and synthetic corundum and of synthetic sapphire and strontium titante. Other stone combinations have been developed, particularly opals, imitations of emerald and some of the diamond imitations. Saltwater cultured pearls are immersed in a very light pink dye to give them an appealing rosy tine that makes it much easier to match colors for uniform necklaces.
All black onyx starts out a much lighter gray.
With dyeing it becomes the familiar dramatic black that jewelers refer to as black onyx. Many pieces of lapis lazuli, for example, have a patchy blue color that dyeing evens out. Other popular gems that are sometimes dyed are very low grades of coral, emerald, jadeite, opal and ruby Foil Backing -has been used to add color and brilliance to gems for over 4000 years. As gem-cutting techniques progressed and brought out more brilliance in stones, these backings became less popular. Today oil backings are occasionally found on corundum cabochons, but they are more likely to be seen on glass imitations of ruby and sapphire.