Yellow Opal

Yellow common opal is reported from many places. Geology.com names Australia and Nevada for yellow opal, notes that Nevada material ranging from bright white to buttery yellow is subject to crazing and color change after mining, and lists yellow opal from Oregon. CIBJO records yellow common opal in Brazil (Piauí, Pará and Rio Grande do Sul), a colorful yellow variety from Kenya, yellow-green opal from Mali, translucent yellow-orange opal in Western Australia, and yellow or green opal nodules in Turkey.

Yellow opal has opal's low refractive index (about 1.44 to 1.46) and low specific gravity, and no play-of-color. Some yellow opal fluoresces; Nevada material often glows bright green under ultraviolet light, attributed to traces of uranium. Chalcedony, a common look-alike, is harder (Mohs 6.5 to 7), denser (2.58 to 2.62) and has a higher spot refractive index (1.53 to 1.54). Mindat treats names such as "lemon opal" as unnecessary names for color varieties of opal.

Opal is fairly soft and brittle and contains water, so it may crack after sudden or extreme changes in temperature or humidity. CIBJO advises protecting opal from abrasion, thermal shock and harsh chemicals, keeping it away from direct sunlight and other heat sources, and cleaning it with a damp cloth only; ultrasonic cleaners should not be used.

The Japanese retailer Pascle gives yellow opal the meanings it associates with opal in general, saying it is traditionally believed to encourage creativity, and presents it as a charm for love. Pascle lists opal among October birthstones in Japan. These are retail traditions, not verified properties.