Sapphire: Properties, Origins, Treatments and Value

Sapphire is corundum in every colour except red. The word alone means blue, and blue is what the world pays for, but the same mineral gives pink, yellow, green, purple, orange, colourless, and the pink-orange the trade calls padparadscha. Chemically it is the same stone as ruby, aluminium oxide with a different set of guests in the lattice, and the theory of sapphire is the theory of which guest arrived, in what company, and how much iron came with it.

What it is

Al₂O₃, trigonal, hardness 9, no cleavage, refractive index 1.762 to 1.770, birefringence 0.008, specific gravity around 4.00. Sapphire is pleochroic, showing two shades of its colour along two crystal directions, and it grows in bands, so colour zoning is common: straight bands of blue against near-colourless are the signature of Sri Lankan rough. A cutter orients the table to spread a single zone across the face, which is why colour is judged face-up and the zoning is found by looking through the side. Corundum also forms with fine needles of rutile, the silk that softens Kashmir blue and that heat dissolves.

Where the colours come from

Blue is an intervalence charge transfer: iron and titanium adjacent in the lattice, passing an electron, absorbing yellow and transmitting blue. The mechanism is efficient, so very little of either element makes a strong colour, and too much iron does other things: it pushes a stone toward green or yellow on its own, and passing charge between its own states it darkens the blue until a stone closes up. Pink and red are chromium. Yellow is iron and colour centres. Padparadscha is a chromium pink with an orange component that can be natural or, in the notorious cases, diffused. Colour-change sapphire, blue in daylight and violet under a lamp, is vanadium. Hue, tone and saturation describe the result, and saturation without grey is what the price follows in every colour.

Where it comes from

Kashmir, a deposit above four thousand metres in Padar opened around 1881 and exhausted within a decade, produced the velvety, sleepy blue that carries the largest origin premium in coloured stones; what trades today is old material resurfacing. Burma, from Mogok, gives a deeper, rounder royal blue. Sri Lanka, the classical source, works the gem gravels of Ratnapura and Elahera and gives the bright, open Ceylon blue as well as most of the world’s padparadscha and fancy colours. Madagascar, since the Ilakaka rush of 1998, supplies enormous volume of the same metamorphic family, so close to Ceylon in the hand that separating them is one of the harder origin calls. Thailand, Cambodia and Australia produce iron-rich basaltic sapphire that reads dark. Montana gives small, clean, distinctive stones from Yogo and Rock Creek. Nigeria, Ethiopia, Tanzania and Vietnam each contribute.

Origin separations rest on inclusions and trace-element chemistry read against reference collections, and they are opinions. The Kashmir premium sits entirely on such an opinion, which is why important stones travel to more than one house.

How it is judged

Colour, in daylight, against master stones. For blue the trade’s reference points are cornflower, a bright medium blue, and royal, a deeper saturated blue with a hint of violet; the laboratories grant royal blue as an appellation against harmonised criteria, as they do pigeon’s blood for ruby. Velvet, the Kashmir look, is not a colour but a scattering effect from fine silk that spreads colour evenly and softens the mirror return. Then the face-up behaviour through the tilt, because a stone must hold its colour as it moves rather than present at one angle. Then clarity, where sapphire is expected cleaner than ruby. Then weight: fine sapphire reaches sizes ruby does not, so the price-per-carat curve is gentler, but it still steps at five and ten carats and climbs steeply for unheated stones of top colour.

Treatments, and what to demand on paper

Heat is the default. Milky geuda rough, carrying titanium locked in silk, is taken to high temperature in a reducing atmosphere and comes out blue and transparent, which is how blue sapphire stopped being scarce. Heat is permanent, stable and legitimate when disclosed; a laboratory reads it in melted silk, tension fractures around crystals and healed fissures. Beryllium diffusion, which reached the market early this century, drives a light element into the lattice and manufactures orange and padparadscha colours; titanium diffusion paints blue onto the surface of pale stones. Both are detectable and both are fraud when undisclosed. Glass filling appears on low grades. The report must state identity, treatment and origin in that order, and the unheated line is the one that moves the money.

The market

Sapphire has two record books. The record total belongs to size: the Blue Belle of Asia, 392.52 carats, sold in Geneva in 2014 for around seventeen million dollars. The record per carat belongs to Kashmir, in stones a tenth the size. Between them sits the whole market: Burmese and Ceylon royal blues with Swiss paper, padparadscha where two houses agree on the name, fancy colours priced by saturation, and a vast commercial tier of heated Madagascar and Sri Lankan blue at prices the ground would never have allowed. The unheated premium runs thirty to fifty percent for fine stones and widens with size and origin.

Care

Hardness 9, no cleavage, good toughness: sapphire is made for daily wear, ultrasonic cleaning and the jeweller’s torch. Diffused stones can lose colour if repolished, because the colour is a skin. Glass-filled stones fail under heat and acids. An unheated sapphire is as durable as any object a family owns.

Where I stand

I buy sapphire unheated only, from Sri Lanka, Madagascar and Burma at origin, and Kashmir when an old stone surfaces with paper that two houses accept. The Ceylon guide and the Ilakaka note set out the two markets I work most; the padparadscha guide sets out the one where a single word opens the widest price gap in corundum. In every case the order is the same. Colour in daylight, then the paper, then the price.

Questions collectors ask

What makes a sapphire blue?

Iron and titanium sitting next to each other in the corundum lattice, passing an electron between them. The mechanism needs very little of either, which is why the finest blues come from low-iron metamorphic deposits such as Kashmir, Burma and Sri Lanka, while iron-rich basaltic sapphire reads inky.

Are most sapphires heated?

Yes. The overwhelming majority of blue sapphire on the market has been heated, much of it from milky geuda rough that only becomes blue in the furnace. Heat is permanent, stable and legitimate when disclosed, and it is why unheated is a price rather than a compliment.

What are the most valuable sapphire colours?

Velvety Kashmir blue and the royal and cornflower blues of Burma and Sri Lanka lead, followed by padparadscha, the pink-orange appellation that laboratories grant against differing criteria. Fine pink, yellow and colour-change stones have their own markets. In every colour, saturation without grey is what the price follows.