Structure : Amorphous
Hardness : 6
Specific Gravity : 2.1
Refractive index : 1.37 - 1.4
Doubly Refracting (bi-refraction) : none
Fire opal glows with the fire of the sun: hot yellows, oranges, and reds so bright they look as though they might glow in the dark. Fire opal sometimes does have play of color like other opals but it does not need this to take a starring role in jewelry. Its juicy color is just the right accent to earth tones or black and also looks great paired with other bright tones.
Unlike most opal, fire opal is often faceted, so you can choose sparkle as well as color. Because it is light as well as bright, fire opal is especially good for earrings, where even small sizes have a big punch of color.
Fire Opal was born in fire, in the ancient volcanoes of Mexico. Fire opal forms when water seeps into silica-rich lava, filling seams and hollows. Under heat and pressure, the silica forms a solid gel, trapping the remaining water within its structure. Small pebbles of fire opal are found embedded in lava flows.
Fire opal that displays play of color but is rare because volcanic opal forms relatively quickly and the spheres of silica rarely have time to settle into the diffraction grids that create play of color.
Fire opal is mined in the Mexican states of Queretaro, Hidalgo, Guerrero, Michoacan, Julisio, Chihuahua and San Luis Potosi. The most important mines in Queretaro were discovered in 1835 and are still producing today. Small quantities of fire opal can also be found in Oregon and British Columbia in Canada.
Fire opal, like all opal, has a high water content. As a result, it should be protected from heat and prolonged exposure to strong light, which could dry it out. Dealers cure fire opal by drying it before cutting to make sure that any instability can be eliminated.
All opal is relatively soft and should be in a protective mounting if set in a ring. Be especially careful with the points of marquise and pear shapes. Clean with mild dish soap: use a toothbrush to scrub behind the stone where dust can collect.