Opal: The October Birthstone and Australia's National Gemstone

The October birthstone is opal, with tourmaline as the modern alternate

October is one of the months carrying two stones. Opal is the traditional October birthstone and the one most people mean when they use the phrase. Tourmaline sits alongside it on the modern list, and is a fair second answer for someone who prefers a single saturated colour. If you are buying a birthday gift and want the stone with the longer association, buy opal.

Opal is Australia's national gemstone

Opal was proclaimed Australia's national gemstone in 1993 — formal recognition of a stone this country supplies to the rest of the world. Australia has historically produced the large majority of the world's precious opal, from a small number of remote fields whose names now function as shorthand for what a stone will look like.

For an October birthday given in Australia, that matters: the birthstone and the national stone are the same stone, and it was dug out of the ground a few states away rather than shipped in. Every piece in our Australian Opal Collection is Australian-made, and the pieces described as solid opal are solid.

Why opal, of all the gems

Opal's colour is not really a colour. Opal is hydrated silica, and inside a precious opal, microscopic silica spheres sit in a regular array. Light diffracting through that array produces play-of-colour, the flashes that shift and reorder as the stone moves. Sphere size influences which colours appear, which is why one stone reads blue and green while another shows red and orange.

The practical consequence for a gift is that no two opals are the same, and the difference between two stones is obvious to anyone holding them. A ruby is judged against other rubies on a scale a non-specialist cannot see; an opal announces itself. The recipient can see exactly why that one was chosen.

What makes Australian opal distinct

Two things, mainly. The first is body tone — the darkness of the background behind the play-of-colour, which does much to set an opal's character and price, though a brilliant stone on a light body can still outrank a dull dark one. Australia's fields produce the full range, including the dark body tones that make colour appear most vivid. Black opal comes principally from Lightning Ridge in New South Wales.

The second is boulder opal, which is effectively a Queensland material. There, opal forms in seams and pockets running through ironstone, and cutters leave the host ironstone attached to the back of the finished stone. The result is a natural dark backing on a solid, single-piece gem, in outlines the rough dictates rather than calibrated ovals. If you want an opal that looks like nothing else in the room, start with Boulder Opal.

The four types you will meet, at a glance

• Black opal — dark body tone, principally Lightning Ridge, New South Wales. The dark background makes play-of-colour read at its most intense, and dark stones are generally the rarest.
• Boulder opal — Queensland opal cut with its ironstone host left on the back. Strong colour on a dark backing, in irregular shapes.
• Crystal opal — a transparent to translucent body with play-of-colour suspended inside it. Held to the light it behaves quite differently from an opaque stone.
• White or light opal — a pale body tone, principally Coober Pedy and the South Australian fields. Gentler, more pastel fire, and the version most people picture when they hear the word opal.

Cutting across all four is the distinction that matters most on a receipt. A solid opal is one natural piece. A doublet is a slice of opal bonded to a dark backing. A triplet adds a clear cap over a doublet. None of these is a fake, but they must be disclosed, they are worth less than solid stones, and they are more vulnerable to water and long immersion. If a listing does not say solid, ask. Our guide to opal types explains how to read a side profile for layer lines.

What makes a good October gift, and what to avoid

Start with wear, not with the stone. Opal sits at about 5.5 to 6.5 on the Mohs scale, below quartz at 7, which means everyday grit can scratch it. It has no cleavage, but it dislikes knocks and sudden temperature change. Earrings and pendants live an easier life than rings, so if the recipient works with their hands, gardens, or simply forgets to take jewellery off, Australian Opal Earrings are the more forgiving gift. If a ring is the right answer, choose a protective setting rather than a high, exposed one.

Then judge the stone. Body tone sets the class of stone, but brightness of play-of-colour can override it: a bright stone with a common colour beats a dull stone with a rare one. After that, look at how much of the face actually shows colour, the pattern, and the colours present — red is generally the rarest, then orange, with green and blue the most common. Check for cracks and sand inclusions. Note that opal is sold by the piece as often as by the carat, because face size and shape matter more than weight; do not shop it the way you would shop a diamond.

Things to avoid: a bargain sold as black opal, which may well be a doublet or triplet; buying from a single photograph taken at the one angle where the stone fires; and ordering a surprise ring without knowing the finger size. If a ring is what you want, Design Your Ring is the sane route, and it allows the stone to be chosen first.

On budget, the honest position is that a modest opal chosen well looks better than an expensive one chosen badly. Our Australian Opal Under $250 and Australian Opal Under $500 collections are a sensible place to start for a birthday gift, and the selection criteria are the same at every level.

One care point that is not optional

Never put opal in an ultrasonic or steam cleaner. Not the machine at home, not the one on a jeweller's counter during a routine clean. Opal contains water, commonly a few per cent and sometimes as much as twenty, and the combination of vibration, heat and thermal shock is exactly what it cannot take. Doublets and triplets add a second failure point, because water can work its way between the bonded layers.

Warm soapy water and a soft cloth, then dry it. Store it away from harder stones so it is not scratched in a box. Say it out loud when you hand the gift over: the ultrasonic cleaner is the real hazard, and it is usually reached for with the best of intentions. Our opal care guide covers long-term storage and everyday wear in more detail.

Buying an opal you can stand behind

Ministry of Gems is a three-generation family gem house with roots in the Ceylon gem trade from the 1960s, working from its own mines through cutting, grading, design and manufacture. Every piece is assessed in-house by an FGAA-qualified gemmologist, and certification is available on request. Australia-wide shipping is free and insured, returns run for 30 days, and re-authentication is free for the life of the piece; the terms are set out under Our Guarantee. If you are buying for an October birthday and want a second opinion first, ask. That is what the gemmologist is for.

Common questions