Hue, Tone, Saturation: How Gemstone Colour Is Graded

Ask ten people to describe the colour of a ruby and you will get ten moods: deep, rich, warm, blood-red, wine. None of them is a measurement. The trade and the laboratories describe colour on three axes, and once you can see the three separately, most of the mystery of gemstone pricing disappears. Colour is hue, tone and saturation. The price lives mostly in the third.

Hue

Hue is the name of the colour: red, purplish red, orangey red, blue, violetish blue, greenish blue. Laboratories place it on a wheel of named positions and describe a stone by its dominant hue with a modifier for the direction it leans. For ruby, a slight purplish lean is not a fault; the finest Burmese stones carry a hint of blue that makes the red look cooler and cleaner, and it is part of what the pigeon’s blood appellation describes. Orange is the enemy: it shows at three metres, it reads brown under a lamp, and the market discounts it at once. For blue sapphire the enemy is green, which arrives with iron and turns royal blue into something the trade calls steely. Hue matters at its edges, where the modifier tips over from a nuance into a different stone.

Tone

Tone is how light or dark the colour is, from nearly colourless to nearly black. Most fine stones sit in the middle of the scale, medium to medium-dark, and the errors are at both ends. Too light and a ruby is a pink sapphire, a lovely stone at a different price. Too dark and the stone reads rich on a white tray in daylight and dies at dinner, because there is no light left inside it to reflect. Clients often ask for dark when they mean saturated, and I spend a good part of the first conversation separating the two. A stone to be worn in the evening needs a lighter tone than one judged only in daylight, which is why purpose belongs in the mandate.

Saturation

Saturation is the purity of the colour: how much of it is the hue itself and how much is a grey or brown mask laid over it. The scale runs from greyish or brownish at the bottom to vivid at the top, and it is where the money is. Two rubies of the same hue and the same tone, one with a grey mask and one without, are separated by a multiple, not a percentage. The grey one sits on the tray; the vivid one lights up. This is what the trade means by a stone that lights up, and it is why the seller’s lamp is warm: incandescent light hides brown and adds glow, and a stone of poor saturation borrows fire it does not own. Daylight at 5500 K shows the mask. Judge there.

The fourth axis: the window

Colour is graded on three axes, but a stone is seen through its cut, and a shallow pavilion opens a window: a region in the middle where light passes straight through instead of returning, so the centre is not dark but dead. Hold the stone over print; if you can read through it, the cutter chased weight. A windowed stone of vivid saturation reads as a stone of moderate saturation to anyone not holding it in a lamp, and recutting is the arithmetic of trading weight for light. I count the window as the fourth axis because in practice it changes what the eye sees on the other three.

Training the eye

Colour memory is a myth, mine included. The eye is trained by comparison, not by looking harder. Master stones, a row of graded reference stones of one species, are the instrument: a ruby beside a master tells the truth in about four seconds, and a ruby judged against the ruby you remember from last month tells you a story. The second instrument is distance. Colour that holds at three metres is colour; colour that needs the loupe is a description. The third is a fixed light, carried with you, because every room in the trade has a lamp chosen to flatter what is sold in it.

Hue at the edges, tone in the middle, saturation above all, and the window because it lies about the other three. Say those four words to a dealer and watch the price of the parcel change before he opens it.

Questions collectors ask

What is the most important factor in gemstone colour?

Saturation. A stone of the right hue and a medium tone with grey or brown in it never lights up, and the market prices that first. Tone comes second, because too dark reads black in the evening and too light reads pink or pale. Hue matters at the edges, where orange creeps into red or green into blue.

What does vivid mean on a laboratory report?

Vivid is the top of the saturation scale: colour with no visible grey or brown mask, at full strength for the hue. Laboratories use it sparingly and the trade words that carry premiums, such as pigeon's blood and royal blue, all sit at or near vivid saturation in a medium to medium-dark tone.

Does a gemstone's colour change with the light?

Yes, and that is why colour is judged in a defined light. Warm incandescent light flatters red stones and wakes the fluorescence of Burmese ruby; daylight at around 5500 K is the reference and shows the brown or grey a lamp hides. Some stones change hue between lights; most simply change in tone and saturation, and the change is part of what you are buying.