Opal Buying Guide — How to Examine and Evaluate a Precious Opal
This is a hands-on practical guide to assessing the qualities of any precious opal. You don't need to be an expert — the very nature of opal is that, barring imitations, simulants, synthetics and serious faults, any non-expert can compare two stones side by side and tell which is the better one.
What follows is the framework we use to look at every stone that comes across our bench. Work through it in order; with a bit of practice and a 10× loupe you will assess an opal as well as most dealers do.
OPAL is a PHENOMENAL stone. Find out why in the text below.
1. Is it a genuine, natural, untreated opal?
Before any quality assessment, confirm what you are actually looking at. Five separate questions:
- Is the material opal at all? Not a simulant or a matrix-only piece.
- Is it natural, not synthetic? Synthetic opal is chemically close to natural but is laboratory-grown, typically with a resin binder — making it more accurately a simulant. (See our types of Australian opal guide.)
- Is it entirely opal? Not a doublet (thin opal slice on a dark backing), a triplet (doublet with a quartz cap), or a mosaic of small pieces glued together.
- Check the back and sides. Make sure the back is opal material — i.e. a homogeneous stone. In the case of a boulder opal, check the point where the ironstone meets the opal to ensure it is a natural join, not something adhered to a backing. A solid stone shows no glue seam at the sides — they can be tricky.
- Is it free of treatments? African opals (usually hydrophane) are sometimes impregnated to change body colour or to slow crazing. Australian opals are known for their stability and it is very uncommon for any treatment other than cut and polish. Any treatment must be disclosed at sale.
2. What flaws does it have?
Use 10× magnification or stronger. Under a loupe you don't expect perfection — you expect to be able to identify any flaw and make an informed decision about it.
Sand and holes
Sand on the face can sometimes be polished out, sometimes not — depending on whether the limitation is the cutter's or the stone's. Sand or holes on the back are generally acceptable in moderation, but large or deep ones affect translucency, clarity and structural integrity, and need to be reflected in the price. Where exactly the sand or holes sit matters — can they be hidden in a setting?
Check that any holes have not been investigated (drilled deeper to gauge depth) and then refilled with resin and sand. A polished refill can look natural to the naked eye but will show under 10×.
Dome height — flat vs high
The height of the dome is usually dictated by the rough. Higher domes carry higher per-carat prices because they require thicker, more solid colour-bearing material — typically rarer “nobby”-style rough, in contrast to the more common seam material that yields flatter stones. Sometimes a flat top is unavoidable because the colour layer in the rough is thin and a higher dome would grind through it. A flat top is not in itself a fault but, since high domes are prized, it must be reflected in the value.
“Windows”
If the front of the stone is translucent and the back is opaque, the opaque back is part of what gives the front its appearance. If the cutter has accidentally ground through the back in one spot, that spot becomes a window — a translucent area that lets light through and breaks the consistency of the body colour. Many stones have windows. The same stone without a window would command a noticeably higher price.
Cracks
Cracks are best viewed under microscope or magnifier; for crystal stones, hold up to sunlight. Beware of filled cracks — resin in the crack makes it harder to spot with the naked eye, but the crack is still visible to a loupe. Whether a crack is natural, induced or inflicted; how it is positioned; whether it can be removed by recutting — all of these affect the value, anywhere from a slight discount to almost total loss.
Chips
Check for chips on the face and the back. Also check that no chip has been glued back in place — under a loupe, the glue line will give it away.
Ginger whiskers and feathers
Some stones contain natural crack-like inclusions known as ginger whiskers or feathers — usually orange-brown, with naturally deposited foreign matter inside. Where possible these are cut out of the stone at the cutting stage; where not, they affect the gem's perfection and its price.
Growths
Growths look a bit like dirty grey or off-white liquid spilled internally on the stone — murky grey clouds under magnification. They can “grow” outward and cover the colour over time. Growths are field-specific — only a few fields have problems with them, and avoiding stones from those fields is the most reliable defence.
Lines and webs
Grey or black lines — straight or squiggly, running through or across the face — mostly subtract from value. They can occasionally form a pleasing pattern in their own right, but on an otherwise outstanding stone a network of lines is a deduction.
A web is a variation on lines that resembles a spider's web sitting in the opal, limiting the visible fire. Webs are an obvious issue under magnification but can be mistaken at first glance for a simply duller fire.
Thick colour-separation borders
Between adjacent colour plates (the boundaries between arrays of silica spheres that produce the play-of-colour), there can be dark borders. Absence of borders is usually preferable. Where borders are subtle they can be a pattern feature of their own; where they are thick they take away precious surface area that would otherwise be visible fire.
Colour separations that look like cracks
At a colour-plate boundary there is sometimes an effect that visually resembles a crack. Magnify to confirm it is not one. Even when it is not, the appearance alone can affect the value — an untrained buyer may mistake it for a crack.
Foreign inclusions
Inclusions can be of any kind. In a translucent stone you must consider any that are visible from the front under any lighting condition.
Consistency
Consistency in translucency, body tone, body colour, visible play-of-colour and pattern tends to be perceived as perfection. Inconsistent features subtract from beauty for most buyers — though “most” is not “all”; this one is the eye of the beholder.
Non-opal host attached to the stone
Check the back, sides and edges. A thin layer of host matrix is generally acceptable; thicker amounts can crack the opal over time because the host expands and contracts at a different rate from the opal itself. Boulder opal is a special case — the ironstone host is integral to the gem (see our types of Australian opal guide).
3. Is the cut and polish of high quality?
The cutter's decisions interact heavily with what the rough allowed. Look at:
- Shape proportion. Even and well-proportioned. Has the cutter saved carats by leaving a badly proportioned outline, or by failing to remove dead spots (less colourful areas) that would have given a cleaner shape?
- Dome profile. Evenly proportioned left-to-right, with some cabochon — not unnecessarily flat. (See the dome reference in our classification tables.)
- Flat spots on curved surfaces. Often left in place because correcting them would lose carat weight; a perfect surface costs more carats and should command more money.
- Surface finish. Free of scratches and grind marks.
- The back. Sometimes left sanded (intentionally, to reduce translucency, or as cutter style); sometimes polished shiny. Either is fine if even — lumpy backs add carat weight that the buyer is paying for, and may force a recut to seat in a setting.
4. Does the stone “talk” to you?
When you pick up the stone and tilt it slightly in your hand, the best opals produce their full play-of-colour with only a small tilt — the stone “talks” readily. A stone you have to roll all the way over to see its colours is not talking the way a great stone does.
Pattern size
Small or pinfire pattern is generally less prized than large pattern. Sometimes a stone has overlapping small and large pattern, which can be more interesting than the large pattern alone.
Directionality
Turn the stone through a full 360 degrees and look for directional fire — whether the colour-play is visible only from certain angles. A directional stone needs to be mounted with its strongest face up; a stone that is non-directional (bright from every angle) is generally more valuable. If the stone is destined for a pendant, hold it vertically and turn it through 360 degrees as well.
Special patterns
Rolling flashes, harlequin, ribbon, cat's eye and Chinese writing patterns all add scarcity and value — especially when the pattern is large, obvious and beautiful. True harlequin is genuinely rare and may multiply a stone's per-carat price.
Fire intensity
Fire intensity is likely the single most influential characteristic on opal value. The right way to assess it is side-by-side comparison — placing a really good stone beside a great one can make the good one look dull. A great stone may be double the per-carat price of a stone that is already not cheap. (See our body tone guide for how brightness interacts with body tone.)
Colour range
How many of the spectral colours are visible? More is generally better, unless you are specifically pursuing a colour effect (e.g. a pure red, a deep blue-green play).
Mobile, top-facing colour
The fire should be visible from the top of the stone and should be mobile, not static. Static colour defeats the purpose of an opal, which is famous for its dynamic moving play-of-colour. You should not have to turn the stone almost onto its side to get the colour to play. This is where you learn why opal is a phenomenal stone: the play-of-colour in opal is, in gemmological terms, literally a phenomenon. So it truly is a phenomenal stone.
Multiple lighting conditions
View the stone in several light environments before committing:
- Direct sunlight
- Cloudy / overcast daylight or in the shade
- Incandescent indoor light
- Fluorescent light
- Low-light indoor (typical dinner-party lighting)
Some stones look spectacular under direct sunlight but die indoors; others sing in overcast or in shade. The most prized stones perform across every condition, and especially under low evening light — the environment in which opal jewellery is most often worn.
Tools to take with you
- A 10× jeweller's loupe at minimum. Higher magnification is even better for confirming cracks, refills and inclusions.
- A torch for backlighting crystal stones.
- A reference stone if possible — side-by-side comparison beats absolute judgement every time, in our experience.
- Access to several different lighting conditions, ideally including natural sunlight.
A note on side-by-side judgement
The single most useful trick in opal evaluation is the side-by-side comparison. The eye is poor at absolute judgement of brightness, body tone or pattern; it is excellent at relative judgement. If two stones look similar in isolation but one is clearly better when held against the other, trust the comparison. Reputable dealers do the same exercise constantly when grading their own stock.
This guide was put together by George Emmanuel Christianos — it's not perfect but it should give you a good starting point…
Where Christianos Opals fits in
Every stone in our inventory has been examined by us under exactly this framework before it goes on the books. Each one carries the body tone, dome, brightness, shape and origin we assess from our own bench, and is photographed front and back so a buyer can do their own loupe-equivalent check.
If you have any specific questions about a stone in our catalogue, or if you would like our view on a stone you have purchased elsewhere, email us.