What makes an opal valuable? The four things that actually matter

Short answer

An opal's value comes down to four things: colour, brightness, play and depth of colour, and tone. Colour is which colours the stone shows. Brightness is how strongly they show. Play and depth of colour is how the colour moves as the stone turns and how far it runs into the stone. Tone is how dark the stone's background is, from white through grey to black.

The rarest and most valuable combination is a black tone with bright colour. Black opal is rarer than white, and colour shows up more strongly against a dark background, so a bright black opal will usually be worth more than a bright white one.

Why four, not ten

Opal can be graded on around ten factors. We use four.

Someone buying their first opal isn't going to study ten grading criteria, and they shouldn't have to. The other factors are real, but they add detail rather than change the picture. For most customers it comes down to price and quality, and these four tell you about quality. They're also the easiest to judge with your own eyes.

1. Colour

Colour is the flashes of spectral colour you see in the stone. It's what makes an opal precious.

Look at how many colours you can see and how much of the stone they cover. A stone that shows colour across its whole face is worth more than one with a single patch.

Not all opal has colour. Opal with no colour is called potch. It is the same material, but it can't split light into colour.

2. Brightness

Brightness is how strongly the colour shows. Two stones can show the same colours, but one glows and the other looks faint.

Look at the stone in ordinary light at arm's length, not under a bright lamp. If the colour still jumps out, it's bright.

3. Play and depth of colour

Play of colour is how the colour moves across the stone as you turn it. Depth is how far that colour runs into the stone, rather than sitting as a thin layer on the surface.

Turn the stone slowly. Good play means the colour shifts and rolls as it moves, instead of switching on at one angle and disappearing at the next. A stone where the colour goes deep holds its colour from more angles, and it's the difference a miner is pricing when they ask more for a parcel with thick colour bars.

4. Tone

Tone, or body tone, is how dark the stone's background is. It runs from white to grey to black.

Black tone is the rarest, and it's why black opal looks the way it does.

Why black opal stands out

Precious opal is made of tiny spheres of silica, far too small to see, stacked in a regular pattern. That regular stacking acts like a prism: it splits white light into its colours. That's the play of colour.

In black opal, the colour sits on a dark layer. The dark background absorbs the stray light that would otherwise wash the colour out, so what's left is the pure colour, standing out against black. The same colour on a white background looks paler, because the white reflects light straight back at you.

Potch has the same silica spheres, but they're uneven and jumbled. They can't split light, so there's no colour.

Why Lightning Ridge opal costs more

Lightning Ridge is the main source of the world's black opal. Coober Pedy, mined since 1915, is the largest opal field in the world and produces most of its white and light opal, which is the most common type. More supply means lower prices, so white opal is generally cheaper than black opal with good colour.

Lightning Ridge opal is also harder to cut. In our experience it has more pockets of sand running through it than opal from most other fields, so getting a clean finished stone out of the rough takes more work and more material is lost along the way.

The field doesn't set the price

A dark tone is valuable, but it can't create colour that isn't there. A black opal with weak colour isn't worth much, and a bright white opal can outprice it.

The same goes for the field. Some opals from other fields are worth more than Lightning Ridge opals. Whatever the mine, it comes down to the same four things: colour, brightness, play and depth of colour, and tone.

Other Australian opal

Opal is found in other parts of Australia too. Two you may come across:

• Boulder opal from Queensland, where the opal forms in seams and patches inside ironstone. The stone is usually cut with the brown ironstone left on the back.
• Pink opal from Western Australia. This is a common opal, meaning it has no play of colour. It's valued for its soft pink colour rather than for the four factors above.

We don't have our own miners in those fields yet. If you're after one, we can source it through suppliers we trust. Just ask.

Buying rough opal at the mine

We buy directly from the miners, not through dealers. That's how we can name the field for every opal we sell, whether it's Lightning Ridge, Coober Pedy or Mintabie.

A parcel of rough opal looks nothing like the finished stone. It's full of dirt, rock and potch, so you're buying opal in its roughest shape, and most of the time it's a gamble.

What you're looking for is colour showing through. Sometimes there are thick, bright colour bars you can see straight away. Sometimes the colour only shows when you wet the stone. When the colour is bright and easy to see, the miner will ask more for it, because it may run all the way through the stone. Sometimes you get lucky: a piece with little visible colour gets cut and reveals far more than it showed.

We wet every stone and go over it with a gemmologist's torch, looking for cracks, sand spots and other inclusions. A crack can open up once a stone is cut and set, and sand means waste when we cut it. A piece that shows no colour at all is potch.

Rough opal is priced by the carat, then valued on the four factors above, plus whether any colour bars are showing. Other things move the price too. A miner who has just found a new vein might sell a little cheaper because they need the cash to keep digging.

The real value only shows once the stone is cut. That's where it's all worth it.

Staying underground in Coober Pedy

When I visit my miner in Coober Pedy, I stay underground.

Coober Pedy summers regularly go above 40°C, so a lot of the town lives below the surface in homes called dugouts. Many of the early ones were opal mines: miners dug through their claims, and once the opal ran out, they turned the hole into a house. Down there it sits at around 23°C all year, whatever the weather is doing above.

It works in Coober Pedy because of the rock. The sandstone there is easy to dig but strong enough to hold up large rooms. You won't find a town of dugouts at every opal field.

For the geology behind each field, see how opal formed in Australia.

See it for yourself

Every opal we sell is photographed as the exact stone you'll receive, and each one is one of one. Judge the colour, brightness, play and tone yourself.

Browse the Lightning Ridge black opal or see the Coober Pedy stones. If you'd rather choose the setting first, you can design your own and swap the stone later. The bezel takes any of them.

Common questions

What are the four most important things when buying an opal?

Colour, brightness, play and depth of colour, and tone. Colour is which colours the stone shows, brightness is how strongly they show, play and depth is how the colour moves and how far it runs into the stone, and tone is how dark the stone's background is. Opal can be graded on around ten factors, but these four decide most of the price.

Why is black opal more valuable than white opal?

Because it is rarer and its colour stands out more. A dark background absorbs stray light, so the play of colour isn't washed out. Black opal comes mainly from Lightning Ridge, while white opal is the most common type, mostly from Coober Pedy.

What is potch?

Opal with no play of colour. It is the same material as precious opal, but the silica spheres inside it are uneven and disordered, so they can't split light into colour.

Why do people in Coober Pedy live underground?

To escape the heat. Summer temperatures regularly go above 40°C, but an underground home stays at about 23 to 25°C all year. Many early homes were worked-out opal mines, and the local sandstone is easy to dig yet strong enough to hold up large rooms.