Opal's Play of Color

Pick Opal (White) or Opal (Black) from the Material dropdown and switch to Realistic, and the stone stops behaving like every other material in the list. Instead of one body color running through it, you get patches of green, blue, gold and violet across its surface that change as you turn it. This page explains what you're looking at and what you can do with it.

Why opal is its own thing

Almost every other stone in the dropdown gets its color the same way: light goes in, some of it is absorbed on the way through, and whatever survives comes back out as the body color. Ruby is red because ruby swallows the rest.

Opal has no color of its own. It's a stack of microscopic glass beads, packed in neat rows like oranges in a crate. Light coming through hits those rows and gets bounced back in a single pure color — and which color depends entirely on how wide the gaps between the beads are and what angle you're looking from. Turn the stone a few degrees and a green patch goes gold, or blinks out completely and something behind it lights up instead.

Real opal isn't one tidy stack, either. It's a mosaic of small patches, each with its own bead spacing and its own tilt. That's why a real opal flashes in irregular blocks rather than glowing all over at once, and it's why no two opals look alike.

What you'll see in the render

Gem Diagram builds the stone out of those patches and works out, for every ray it traces, which color that patch would throw back. So:

• Rotate the stone and the colors move. Not a highlight sliding across a fixed color — the colors themselves change. This is the whole point of opal, and it's the reason a still screenshot never does one justice.
• Some patches stay dark. Real opal is part precious, part common — the dull material cutters call potch. Roughly a quarter to a third of the patches here never flash, which is what keeps the stone from looking like confetti.
• The color sits on the shell, not in the depths. The bead stacks that flash are the ones just under the surface, so the color is thrown straight back at you rather than gathered from somewhere inside. This is why the play of color survives even on a stone you can't see into at all.
• The body is milky, not glassy. Opal scatters light rather than passing it cleanly, so it reads soft and translucent. That haze is deliberate, and it's the Opacity slider doing it — see below.
• There's no fire. Fire is white light being split into a spectrum by the stone, and opal doesn't do it — its dispersion is zero. Every color you see is play of color, a completely different effect that happens to look vaguely similar in a photograph.

White opal and black opal

The dropdown carries both, one after the other, and they are the same stone optically — same silica beads, same RI of 1.45, same flashes. What changes is the potch the play of color sits on.

• Opal (White) has a milky pale base. The flashes sit on a bright body, so they read as pastels, and the stone as a whole is light and translucent.
• Opal (Black) has a near-black base — a very dark gray, often blue-leaning, what the trade calls an N7–N9 body tone. The flashes are the same colors, but against a dark body nothing dilutes them, so they read far more vivid. This is the whole reason black opal is the valuable one.

Black opal also opens denser: Opacity 0.92 against white opal's 0.75, and a slightly lower specific gravity (2.10 vs 2.15), so an identical cut weighs a touch less.

Switching between the two is just picking the other entry in Material — the geometry, the size and the cut are untouched, so it's a fair side-by-side.

The Opacity slider, and opal's default

Opal is the one material that doesn't start at zero opacity. Its Opacity row in the Material card opens at 0.75 for white opal and 0.92 for black — well toward the opaque end, because a precious opal is translucent, not glassy. Light glows through a thin girdle, but you never get a clean view of the far facets through the table.

That's a starting point, not a lock. Drag it right toward see-through for a water opal or a thin crystal opal; drag it all the way left for a dense white or black body. The play of color doesn't fade as you do — it comes off the shell, so a completely solid opal still fires. Only the milkiness moves.

Once you move the slider, the design keeps your value. Opal's 0.75 fills in only while a design has never set one, so it can't override your choice on reload. See Gem Color & Color Depth for how the control works on every other material.

The color picker is off

Select Opal and the Color row grays out. There's nothing to set: the material supplies its own milky base, and the flashes on top of it come from the geometry of those bead stacks, not from a hex value. Setting a body color would have nothing to attach to. Opacity stays live, though — it's the one control that still shapes how an opal reads.

The one other material that does this is Chrysoberyl (Alexandrite), for a completely different reason — see Pleochroism (Alexandrite). Alexandrite changes color as you turn it because its crystal absorbs differently along different directions. Opal changes color as you turn it because of the bead spacing. Same gesture from the viewer, unrelated physics.

What this means for cutting

Opal is soft — 5.5 to 6.5 on the Mohs scale, well below quartz — and it's more often cut as a smooth dome than faceted, precisely because facets don't add much to a stone whose color doesn't depend on total internal reflection. It also weighs noticeably less than it looks: at a specific gravity of 2.15, an opal comes in at roughly 60% of the carat weight of the same design cut in corundum. Watch the stone report if you're pricing by weight.

If you are faceting it, angles still matter, just not for the usual reason. The pavilion isn't there to bounce light back at your eye — it's there to show you more patches at more angles. Shallower work tends to show broader sweeps of a single color; more facets at more angles break it into finer, busier flashes.