Spinel: Properties, Origins, Treatments and Value

Spinel is magnesium aluminium oxide, a cubic mineral that grows as sharp octahedra in the same marble that grows ruby, and for most of the history of the trade nobody separated the two. The Black Prince’s Ruby in the British Imperial State Crown is spinel. So is the Timur Ruby. Mineralogists made the separation by reading structure instead of hue, and by then the stone had spent a thousand years being priced as ruby and the next two hundred being priced as its understudy. It is neither. It is one of the most honest gems in the trade, and the market has spent this century finding that out.

What it is

MgAl₂O₄, crystallising in the cubic system as octahedra, often as the flattened triangular twins the trade calls macles. Hardness 8, no cleavage, good toughness. Refractive index near 1.718, rising with iron and zinc, and singly refractive: one index, no birefringence, no pleochroism. That single fact governs the cutting. Corundum is doubly refractive, and a cutter must set the table close to perpendicular to the optic axis or watch a second colour bleed into the face; spinel has no second colour, reads the same from every direction, and can be oriented for shape and yield alone. It also tends to be evenly coloured, without the angular zoning of sapphire, so what the table shows is usually what the whole stone is. Specific gravity about 3.60. Under the polariscope a spinel stays dark as it turns, with only strain-related flashes, which is how a gemmologist separates it from ruby in a minute.

Where the colours come from

Chromium gives the reds and pinks, and in a low-iron host the chromium fluoresces red in daylight, so the stone looks lit from inside; this is the mechanism behind Burmese red spinel and the neon pinks of Mahenge, and it is the same mechanism that makes Mogok ruby glow. Cobalt, in trace quantity, gives an electric, pure, slightly violetish blue with the grey pushed back; it is measurable, and the words cobalt blue on a serious report describe chemistry rather than praise. Iron gives the ordinary blues, greys, mauves and purples, respectable and quiet. Vanadium and chromium together give some of the orange-pinks. Because spinel takes colour evenly and shows it from every angle, the colour you see at the tray is the colour you will own.

Where it comes from

Burma is the classical source: Mogok for the reds and pinks that were sold as ruby for centuries, and Namya in Kachin State for the hottest pinks, both from marble. Tajikistan’s Kuh-i-Lal, in the Pamirs, gave the historic Balas rubies of the Mughal and European courts, and the name Balas is a place rather than a species. Tanzania became the modern source when Mahenge produced, from 2007, crystals in pinks and reds at a saturation the species had not been assumed capable of; what the deposit looks like now is its own note. Vietnam’s Luc Yen is the origin most associated with cobalt blue, and the market there is small and specific. Sri Lanka gives a wide range from its gem gravels, including greys and blues of some size. Afghanistan, Madagascar and Kenya contribute. Origin carries a premium for Burmese red and for Vietnamese cobalt, and a laboratory can support both from inclusions and chemistry.

How it is judged

Colour, on the three axes, with saturation leading as always. For red spinel the market wants a pure red without orange, and it accepts a slight pink cast that it would not accept in ruby. For pink it wants the neon, the fluorescent glow that Mahenge made famous, and it discounts greyish or brownish pink sharply. For blue it wants cobalt, and it pays a multiple for chemistry that a laboratory confirms. Then clarity, where spinel is usually clean and a clean stone is expected rather than rewarded. Then cut, which for spinel is easier than for corundum and should be judged on light return rather than on orientation. Then size, where the curve steps at five carats for fine colour and turns vertical above ten.

Treatments, and what to demand on paper

Spinel has no treatment industry. Heating is reported on some commercial material and leaves the stone what it was; cobalt diffusion, documented on blue spinel in the last decade, is the one treatment that matters, because it counterfeits the rarest colour in the species and must be stated on any report for a blue stone. The real question is natural against grown. Flame-fusion synthetic spinel has been made for generations, usually in colours nature rarely produces and with a refractive index and density a gemmologist can read; flux-grown synthetic red exists and is more convincing. For anything in the money the report should confirm natural origin, state the absence of treatment, and, for cobalt stones, carry the chemistry. Beyond that, spinel is as honest as goods get.

The market

Spinel spent the twentieth century underpriced because the trade pays for names and spinel had none. That has changed. Mahenge pinks, Burmese reds and above all cobalt blues now trade per carat at levels the species had never seen, and the Hope Spinel, fifty carats from the old Balas source, sold in London in 2015 for close to a million pounds. Fine spinel still sits below ruby of comparable colour, often by a large multiple, and for a buyer who wants the colour rather than the word that gap is the whole case. Liquidity is thinner than corundum, and the stones that hold value are the saturated ones with paper. The buyer’s guide sets out the three colours I hunt.

Care

Hardness 8, no cleavage, good toughness: spinel wears daily without complaint and takes ultrasonic and steam cleaning. There is no oil to dry, no filling to fail and no heat to protect. A natural spinel is as close to maintenance-free as a stone can be.

Where I stand

Spinel is one of the three stones this office hunts on mandate alongside ruby and sapphire, for the reason the theory above makes plain: it gives saturation corundum does not reach in the same hues, it is untreated, and it is still priced below what its quality warrants. I buy Mahenge for the neon, Burma for the classic red, and Luc Yen for cobalt when a stone surfaces with the chemistry on paper. Judged as itself rather than as an understudy, it is one of the best purchases in the trade, and I say so to every client who asks for a ruby they cannot afford.

Questions collectors ask

Is spinel a real gemstone or an imitation?

A real and ancient one. Spinel was mistaken for ruby for most of recorded history because the two grew in the same marble and shared a colour; the Black Prince's Ruby and the Timur Ruby are spinel. Synthetic spinel exists and is common in cheap jewellery, which is where the reputation for imitation comes from, but natural spinel is its own mineral with its own market.

How can you tell spinel from ruby?

Spinel is cubic and singly refractive, so it has one refractive index and no pleochroism, while ruby is doubly refractive with two shades of red along two directions. Under the polariscope spinel stays dark as it turns. A laboratory settles it in minutes, and in the hand spinel reads the same colour from every angle where ruby shifts.

What are the most valuable spinels?

Cobalt blue from Luc Yen, hot pink and red from Mahenge, and the classic Burmese reds from Mogok and Namya, in that order of scarcity. Saturation without grey sets the price inside each colour, and clean stones above five carats of fine colour are rare in all of them.