Unlike traditional gemstones such as diamonds or sapphires, opals are a hardened silica gel with a water content of 5-15%. Opals are prized for their kaleidoscope of colors and have been compared to galaxies, fireworks, and even volcanoes. Australia, Brazil, Mexico, Africa, and the United States are the largest producers of opals since the 19th century. Due to the presence of other colors in unpredictable patterns, no two opals are created equal. However, there are five main categories of opals: White/Light (translucent/white background with several colors), Black (translucent black with several colors), Fire (transparent/translucent with brown, red, yellow, and orange), Boulder (transparent/opaque with several colors), and Crystal/Water (transparent/semi-transparent with exceptional colors).
All opals have a Mohs Hardness of 5.5 – 6.5 and have an amorphous, non-crystalline structure. These stones are formed by spheres of silica and other substances filling the cavities in sedimentary rock cavities or veins of igneous rocks and solidifying. Additionally, opal can mineralize organic substances such as bone, shells, and wood, which is why opals can be referred to as “organic gems”.
The unique “play-of-color” in the stones is due to the layering of small, stacked spheres; as waves of light travel through the spheres, the waves bend and break into the various colors of the rainbow. The size of the spheres determines the color reflected.
Due to their myriad of colors, opals have historically represented change and new beginnings. Romans have admired and described the opals deep, rich colors as early as the 1st century A.D. Other cultures believed opals to possess supernatural powers. Arabic legend states that opals fell from the heavens in lightning. Ancient Greeks believed opals protected the wearer from disease and bestowed the gift of prophecy. Regardless of culture, opals have been widely considered to be the luckiest and most magical due to their color displays.