Diamond: Properties, Grading, Treatments and Value

Diamond is carbon, arranged in a cubic lattice under pressures that exist only a hundred and fifty kilometres down, and carried to the surface in volcanic pipes faster than it could turn back into graphite. It is the hardest natural substance, the most brilliant, and the most thoroughly systematised object in the jewellery trade. This office does not deal in it, and the reasons are part of the theory. Here is the stone as it actually is, without the century of advertising attached.

What it is

Pure carbon, crystallising in the cubic system, most often as octahedra. Hardness 10 on the Mohs scale, an absolute rather than a step: diamond is many times harder than corundum at 9. It is nonetheless not indestructible, because it has perfect cleavage in four directions parallel to the octahedral faces, and a sharp knock on a thin girdle or a pointed corner can split it. Refractive index 2.417, the highest of the common gems, and dispersion of 0.044, which is the fire: white light split into its colours as it leaves the stone. Singly refractive, with no pleochroism. Specific gravity 3.52. It conducts heat better than copper, which is why the simplest testers work, and why a diamond feels cold.

Where the colour comes from

A perfect diamond lattice is colourless. Nitrogen replacing carbon absorbs blue and makes the stone yellow, from the faint tint that drags a grade down the D-to-Z scale to the saturated fancy yellow that is priced the other way. Almost all diamonds carry nitrogen; the rare ones without it, called type IIa, include the most famous colourless stones in history and the ones that fetch the most per carat. Boron makes blue and makes the stone a semiconductor. Pink, red and most brown are not impurities at all but plastic deformation of the lattice, which is why they cannot be grown to order and why natural pink is the rarest colour of all. Green is radiation damage, natural or otherwise, and the laboratories spend a great deal of effort deciding which.

Where it comes from

India supplied the world until the eighteenth century, from the Golconda alluvials that gave the Koh-i-Noor and the Hope; Brazil followed; then South Africa in 1867, where the Kimberley pipes turned diamond from a curiosity into an industry. Today the volume comes from Russia, Botswana, Canada, Angola, the Democratic Republic of the Congo, Namibia and South Africa, with Lesotho notable for very large type IIa stones and Sierra Leone for its history. Australia’s Argyle mine, the source of most of the world’s pink diamonds, closed in 2020, and its stones have appreciated since. Origin is rarely stated on a diamond report and rarely moves the price, which is the first of several ways diamond differs from coloured stones.

How it is graded

The four Cs are the GIA system that the entire trade adopted, and they work because diamond is a single mineral in a single usual colour. Colour runs from D, colourless, to Z, with each grade a small step in yellow tint judged against master stones table-down. Clarity runs from Flawless through VVS, VS, SI and I, judged at ten power. Cut is graded on proportions and finish, and it is the one the buyer can see: a poorly cut stone leaks light through the pavilion no matter its other grades. Carat weight multiplies the rest, and prices step at the round numbers as they do in every stone. For fancy colours the system inverts: colour becomes nearly the whole price, graded on hue and saturation from Faint to Fancy Vivid, and a Fancy Vivid pink of a few carats outprices any white diamond of the same weight by an order of magnitude.

What the system leaves out is what coloured stone buyers spend their lives on. Two diamonds with identical grades are, for most purposes, interchangeable, and that interchangeability is the point: it makes the stone a commodity with a price list. A ruby has no such list.

Treatments and grown stones

High-pressure, high-temperature annealing turns brown type IIa stones colourless and makes fancy colours from ordinary ones. Irradiation makes green, blue and black. Laser drilling reaches inclusions and fracture filling hides them. All are detectable by a serious laboratory and all must be disclosed. The larger change is growth. HPHT and chemical vapour deposition now produce gem-quality diamond at industrial scale, chemically identical to mined stone and separable only by laboratory testing. As production scaled, the price of grown stones collapsed, and the price of commercial mined white diamond followed it down. A grown diamond is a real diamond and an honest product when disclosed. It is also a manufactured object with no scarcity, and it has no resale value to speak of, which is the whole distinction a buyer needs.

The market

Two markets, barely related. Commercial white diamond is graded to a standard, mined in volume, priced from lists, and now competes with grown stones at a fraction of the cost; as a store of value it is poor, and the honest accounting for coloured stones applies to it with the spread made worse. Natural fancy colours of size, above all pink and blue with GIA paper, are a different world: the Pink Star, 59.60 carats, sold in Hong Kong in 2017 for around seventy-one million dollars, and the great blues have sold for tens of millions. Those stones behave like the finest coloured gems, because they are rare in the way coloured gems are rare, and the four Cs cannot capture them.

Care

Hardness 10 resists scratching by everything except another diamond, which is how diamonds scratch each other in a jewellery box. Cleavage is the risk: a hard knock on a girdle or a corner can chip or split the stone, and thin girdles in cheap cuts are where it happens. Fracture-filled stones must be kept from the torch. Otherwise it takes ultrasonic, steam and a lifetime of daily wear.

Where I stand

This is a coloured stone office and I do not trade diamond. The reasons are in the theory: a graded, listed, volume-produced stone with an industry of grown equivalents is the opposite of what I hunt, which is the object that cannot be standardised, cut for light rather than for a grade, and priced on an opinion formed in daylight. When a client needs a fine white diamond I say so and send them to people who do nothing else. When a client asks about a natural pink or blue of consequence, I treat it as what it is, a coloured stone, and the discipline is the same: the stone first, then the paper, then the price.

Questions collectors ask

Are laboratory-grown diamonds real diamonds?

Chemically and physically, yes: they are carbon in the same cubic structure with the same hardness and fire, and only a laboratory can separate them from mined stones. Commercially they are a different product. Their price has fallen steeply as production scaled, and they hold no resale value to speak of, which is the whole distinction.

Which of the four Cs matters most?

Cut, for how the stone looks, because a poorly cut diamond leaks light no matter its grade. Colour and clarity move the price more, in steps the grading system makes visible. Carat weight multiplies everything at the round numbers. For fancy colours the system inverts and colour is nearly the whole price.

Is diamond a good investment?

Commercial white diamond is not: it is graded to a standard, produced in volume, and now competes with grown stones at a fraction of the price. Very rare natural fancy colours, especially pink and blue of size with GIA paper, have set records and behave more like great coloured stones. Everything in between is jewellery.