October has two birthstones — opal and tourmaline — but opal is the older and more iconic of the pair. Named from the Sanskrit upala (meaning precious stone), it has been prized for its play of color since at least the time of Ancient Rome. This guide covers what opal is, how it forms, the main varieties, and what to look for if you are buying one.
Opal is the October birthstone — a hydrated silica mineral (5-20% water) at Mohs 5.5-6.5 hardness, showing play-of-color when light diffracts through microscopic silica spheres. Top Australian black opal from Lightning Ridge runs $500-$10,000 per carat for fine 1.00ct stones; Ethiopian Welo $100-$2,000. Opal also marks the 14th wedding anniversary.
What gives opal its play of color
Opal is a hydrated silica gel that hardens into structured spheres of silicon dioxide. When those spheres are uniform in size and packed in an orderly grid, they diffract light into spectral flashes — the famous play of color that no other gemstone produces. The size of the spheres determines which colors appear: smaller spheres produce blues and violets; larger ones produce reds and oranges. A precious opal with a full spectrum across its surface is exceptionally rare and valuable.
The main types of opal
Australian black opal (mined primarily at Lightning Ridge in New South Wales) has a dark body tone that makes its play of color appear especially vivid. White opal, also from Australia, has a milky body and softer color play. Boulder opal forms in ironstone matrix and is often cut with the host rock left attached. Mexican fire opal is transparent to translucent with red, orange, or yellow body color — sometimes with no play of color at all, prized for the body hue alone. Ethiopian Welo opal, discovered in 2008, has become a major source of bright, affordable opal with intense color play.
Hardness and care
Opal sits at 5.5–6.5 on the Mohs hardness scale — softer than quartz and significantly softer than diamond, which is why opal jewelry needs gentle handling. Avoid ultrasonic and steam cleaning. Clean with a soft damp cloth only. Store opal pieces separately so harder stones do not scratch them. The water content (3–21% by weight) means opal can craze if exposed to sudden temperature changes or very dry environments — store it away from direct sunlight and central heating.
Buying opal — what to look for
Two factors drive value above all others: the brightness and pattern of the play of color, and the body tone. A "harlequin" pattern (large angular patches of color) is the most valuable, followed by broad flash patterns. Pinfire and flagstone patterns are common and more affordable. For body tone, darker is generally more valuable for black opal because dark backgrounds make colors pop. For Mexican fire opal, the body hue itself sets the value — vivid orange-red commands the highest prices. Always check for crazing (fine surface cracks) before buying.
Frequently Asked Questions
Is opal a good choice for an engagement ring?
Opal is beautiful but soft (5.5–6.5 on Mohs) so it requires more careful wearing than diamond, sapphire, or ruby. It can chip or craze if knocked hard. Many couples choose opal for occasion jewelry rather than an everyday engagement ring. If you want opal as an engagement stone, a bezel or protective halo setting reduces the risk, and removing it for manual work, gym sessions, and washing-up significantly extends its life.
What is the difference between solid opal and doublets/triplets?
A solid opal is a single piece of natural opal. A doublet has a thin slice of opal cemented onto a dark backing (often potch or ironstone) to enhance the color play visually. A triplet adds a clear quartz or glass cap on top for protection. Doublets and triplets are much cheaper but cannot be exposed to water for long periods (water can dissolve the adhesive layer). Always ask explicitly whether an opal is solid before purchase.
Are Ethiopian opals as good as Australian opals?
Different rather than worse. Welo opals from Ethiopia often have brighter play of color at a lower price point than Australian black opal, but they are hydrophane (absorb water) which can temporarily change their appearance until they dry out. Australian opals are more stable and have a stronger collector market for resale. For an everyday wear, Australian opal is the safer choice; for a striking color at a lower price, Ethiopian is excellent.
Can you scratch or damage an opal easily?
More easily than harder gemstones. Quartz (Mohs 7) — present in everyday household dust — can scratch opal over time. Routine knocks against hard surfaces can chip or fracture it. With careful wear (off for gardening, gym, swimming, manual chores) opal will last decades. Treated roughly it can be damaged in months. Match the stone to the lifestyle of the wearer.
Choosing opal jewelry at Reve
Opal pendants and earrings are the easiest opal jewelry to wear daily because they take less impact than rings. Browse our pendants & necklaces collection for fine jewelry designs, or visit our New York showroom for bespoke commissions with opal center stones.
