Opal

The clearest sign of precious opal is play-of-color: turn the stone under a light and look for distinct patches of pure spectral color — flashes of green, blue, red or violet — that move, change and switch on and off as the viewing angle shifts. This is different from a simple glow or a single sheen; it is many separate flecks of rainbow color winking across the surface. Common opal has no such display, showing instead a uniform body color with at most a soft, milky internal glow. Because opal forms as a filling rather than as crystals, you will not find the geometric crystal faces that point to quartz or beryl; opal occurs as smooth nodules, seams and cavity fillings, and its surface often has a slightly waxy or glassy feel.

Physical clues help confirm the call and separate opal from harder, denser stones. Opal is comparatively soft at 5.5 to 6.5 on the Mohs scale, so it will not scratch glass cleanly the way quartz does, and a steel point can mark it — though testing should be done cautiously on an inconspicuous spot, since opal can chip. It tends to feel lighter in the hand than its size suggests, a consequence of its water content and porosity, and it has a white streak. A stone that shows true moving play-of-color, lacks crystal faces, feels relatively light and is only moderately hard is very likely opal rather than a glass or plastic imitation.

Opal is classified first by its body color — the background tone against which any play-of-color appears — and this is the basis for its named varieties. White opal has a pale, milky body and is the most familiar type, with play-of-color floating against a light ground. Black opal has a dark gray to black body that makes the spectral flashes appear especially vivid and is generally the most sought-after variety because the dark background showcases the color so dramatically. Boulder opal forms as thin seams within a host ironstone and is cut with that brown host rock left attached as a natural backing, which both supports the often thin opal and lends the stone a distinctive look. Crystal opal is transparent to translucent, letting light pass through the body while still showing play-of-color.

Not all opal flashes color. Fire opal is a transparent to translucent variety with a warm body color ranging from yellow through orange to red; it is named for that fiery body hue, and while some fire opal also shows play-of-color, much of it is prized simply for its glowing orange-red transparency. Common opal, or potch, encompasses the many opals with no play-of-color at all — milky white, gray, green, blue or brown stones used for carving and beads. The orderly internal sphere structure is what divides the two great camps: where the silica spheres are uniform and neatly stacked, the stone diffracts light and becomes precious opal; where they are irregular or randomly packed, the result is common opal with solid color only.

Opal has long been linked to creativity, inspiration and imagination, a reputation that suits a stone whose shifting colors seem to hold a whole spectrum within one body. Across various traditions it has been associated with hope, emotional expression and a kind of inner spark, and in modern crystal-working practice it is often described as a stone that encourages artistic flow and the free expression of feeling. As the October birthstone it also carries gentle associations with renewal and good fortune, and its ever-changing fire has made it a symbol of multifaceted personality and possibility.

These meanings are cultural, historical and spiritual rather than scientifically demonstrated medical effects. Opal is a beautiful and meaningful stone to wear and to keep, but it does not cure, treat or prevent any physical or mental health condition and is not a substitute for professional medical or psychological care.

Opal value is driven above all by the quality of its play-of-color, then by body color, pattern and the form of the stone. Bright, vivid color that covers the whole face of the stone is worth far more than dim, patchy flashes confined to one corner, and a stone that shows a full range of colors — especially including red, which is the rarest to display — commands more than one limited to blues and greens. Body color matters too: against a dark background the colors read as more brilliant, which is why black opal generally sits at the top of the range, while a pale or washed-out body that mutes the flashes sits lower. The pattern of the color, whether broad rolling flashes or a fine mosaic, also influences desirability.

The physical form of the stone is a value factor in its own right. A solid, whole opal is worth more than a thin slice, and this is why doublets and triplets — assembled stones with only a thin layer of genuine opal — are much less valuable than a solid natural gem, and why honest sellers disclose them. Boulder opal is judged with its ironstone backing as part of the stone. Because opal can be fragile and can craze, soundness matters: a stone free of cracks and crazing is worth more than one showing fine fractures. As elsewhere on this site, no specific prices or localities are quoted here; judge an opal by the brilliance and coverage of its play-of-color, its body tone, its pattern and whether it is a solid stone or an assembled one.

The most important distinction with opal is not always natural versus outright fake, but solid versus assembled. A doublet is a thin slice of genuine opal cemented to a dark backing to deepen its apparent color; a triplet adds a clear protective cap on top, so the real opal is only a thin filling in the middle. Both contain real opal, but they are worth far less than a solid stone and can be vulnerable to water seeping into the glue layer. The tell is to inspect the stone from the side: a doublet or triplet usually shows a flat, straight join line and an abrupt change between the opal layer and its backing or cap, whereas a solid opal shows its color extending in three dimensions through the body with no glued seam.

Outright imitations and synthetics also occur. Laboratory-grown synthetic opal is genuine opal in composition but is made by people; it often gives itself away with a too-regular, repeating play-of-color and a characteristic columnar or "snakeskin" pattern when viewed closely, far more orderly than the irregular flashes of natural opal. Glass and plastic imitations are also sold: these tend to look too uniform and perfect, with color that is painted-on rather than truly shifting, and they fail simple physical checks — plastic is much too soft and feels warm and light, while imitation "opal" glass lacks the genuine moving fire. Because real opal is fairly soft itself, hardness testing must be gentle, but a stone whose flashes are suspiciously regular, whose color does not truly move with the viewing angle, or which shows a flat glued layer in profile deserves close scrutiny, and a gemological report is the surest confirmation for an important purchase.

Opal needs gentler handling than most gemstones, for two reasons: it is relatively soft at 5.5 to 6.5 on the Mohs scale, and it contains water. Because of its softness it scratches and chips more easily than quartz or sapphire, so it is best reserved for pendants, earrings and occasional-wear rings rather than everyday knock-about jewelry, and it should be stored apart from harder stones that could scratch it. Clean opal only with lukewarm water, a drop of mild soap and a soft cloth, then dry it gently; this is safe for solid stones, while doublets and triplets should never be soaked, because water can creep into the glue layer and lift it.

The water content brings a special hazard: opal can craze, developing a fine network of surface cracks, if it dries out or is exposed to sudden temperature changes. To guard against this, keep opal away from direct heat, prolonged hot sunlight, hot water and the drying effect of very low humidity, and never use a steamer or an ultrasonic cleaner on it — the heat, vibration and thermal shock can crack the stone outright. Avoid harsh chemicals, and store opal somewhere that is neither baking-hot nor bone-dry. Treated with this modest care, a sound opal keeps its fire for generations; mishandled with heat or vibration, it can be ruined in moments.