Spinel

For most of the recorded history of this trade, spinel was ruby. Dealers bought and inventoried the great red stones in old regalia as rubies because colour and locality were the whole test, and in Burma the two came out of the same marble ground, while the Central Asian workings that gave us the name were spinel throughout. The Black Prince’s Ruby in the Imperial State Crown is the example everyone reaches for, and it is spinel. So is the Timur Ruby. Balas, the old word for it, named a place rather than a species. Mineralogists eventually made the separation by reading structure instead of hue, and by then the confusion had centuries behind it.

What single refraction does to a stone

Spinel is cubic. One refractive index and no pleochroism. There is no true birefringence either — strain will still give you anomalous extinction under the polariscope, but nothing that reaches the eye. The practical consequence shows up in the cutting. Corundum is doubly refractive, so a cutter must bring the table near perpendicular to the optic axis or watch the second pleochroic colour bleed into the face. Spinel has no second colour to fight. The body colour reads the same from every direction, so the cutter can orient for shape and yield rather than hue, and the stone does not change character as you rock it.

It also tends to be evenly coloured. The angular growth banding you hunt for in sapphire is largely absent, so what you read across the table is usually what the whole stone is. In the chromium reds and pinks there is a second mechanism — low iron and strong red fluorescence — and the stone looks lit from inside in daylight.

Material almost nothing is done to

Corundum has a heat treatment industry behind it and emerald an oiling tradition. Spinel has nothing on that scale, though it is not untouched. Heating is reported on commercial material, and cobalt diffusion has been documented in blue spinel — which is precisely the corner of the market where a treated stone costs you most. Ask for disclosure, and for anything in the money ask for a report. But treatment has not rebuilt the category, and that makes it unusually honest goods — provided the stone is natural. Flame-fusion synthetic spinel has been in the trade for generations and flux-grown reds exist, so what I am separating here is grown against found rather than treated against untreated.

The three colours I hunt

Mahenge spinel is the first. The Tanzanian deposit produced reds and hot pinks at a saturation that reset what the species was assumed capable of, and I will set the best of it beside fine ruby. Miners have worked the ground hard since, and what reaches the market now is not what came out early — I have written separately about what Mahenge looks like after the rush.

Neon pink spinel is the second, and it overlaps with the first. The trade has coined several names for the hottest Burmese material, and it is not all one place — Namya in Kachin State as much as Mogok, with the names themselves argued over. The names matter less than the mechanism, which is chromium at good concentration in a low-iron host, fluorescing instead of being quenched. I judge it in daylight, against masters, and I distrust a stone that only performs under a seller’s lamp.

Cobalt blue spinel is the third and the rarest. Ordinary blue spinel takes its colour from iron and reads greyish, sometimes greenish. In a small number of stones cobalt substitutes into the structure and the blue goes electric, pure and slightly violetish, with the grey pushed right back. Cobalt sits there in trace quantity and a laboratory can measure it, so the term belongs to chemistry rather than to praise. Fine cobalt blue spinel in useful sizes is scarce, and Vietnam — Luc Yen in particular — is the origin most associated with it. Burma remains the classic source for red, Tanzania the modern one.

Why it stays underpriced

Spinel is hard enough to wear, untreated, and available at a saturation corundum does not reach in the same hues. It still trades below what its quality warrants. The reason is not gemmological. Buyers pay for recognition, and names build recognition. Spinel has no colour appellation carrying the weight that pigeon’s blood carries in ruby — cobalt blue is the nearest thing, and it describes chemistry rather than grade. Beside ruby it has next to no auction mythology, and it carries a long record of being catalogued as something else.

For a collector that gap is the opportunity and the risk in one. You can buy at the top of the quality range for money that buys nothing near the top in ruby. Here too price per carat still climbs steeply with size, but spinel gives up clean crystals at sizes where ruby has effectively stopped, and that is part of why the money goes further. The risk is liquidity: a thinner pool of buyers means a slower exit, and most people leave that out of the argument for coloured stones as an asset.

My rule is short. Judge it as spinel. It spent centuries standing in for something else and has no need to keep doing it.