Gemstone Colour Grading: How Hue, Tone and Saturation Decide a Stone's Colour

What is gemstone colour grading?

Gemstone colour grading is the way a gemologist describes and compares the colour of a coloured stone such as a sapphire, ruby, emerald or tourmaline. Instead of a single letter, the colour is broken into three parts: hue (which colour it is), tone (how light or dark) and saturation (how strong or pure the colour is). Together they pin down a colour precisely enough that two people can picture the same stone.

This is very different from the D-to-Z diamond colour scale, which measures how little colour a white diamond has. With coloured gems the colour is the whole point, so the question is not "how much colour is missing?" but "how good is the colour that is there?" For most coloured stones, colour accounts for more of the value than clarity, cut or carat weight.

There is no single worldwide standard. GIA (the Gemological Institute of America) teaches a hue, tone and saturation system with 31 hues, a 0–10 tone scale and a 1–6 saturation scale, and many gemologists, including those trained at GIA, use it to describe stones. Other labs, such as GRS, Gübelin and SSEF, use their own wording and sometimes trade names like "royal blue". Most commercial lab reports for coloured gems confirm identity, treatment and sometimes origin, and describe the colour in words, without giving it a letter or number grade.

If you are new to coloured stones, our coloured gemstone guides cover each gem in turn, and any unfamiliar term here is explained in our plain-English jewellery glossary.

What is hue in a gemstone?

Hue is the first impression: blue, red, green, purple, orange and so on. Very few gems are a single pure hue. Most show a main hue with a secondary one, called a modifier. A sapphire might be "violetish blue" or "slightly greenish blue"; a ruby might be "purplish red" or "orangy red".

GIA's system arranges 31 hues around a colour wheel and writes them in a shorthand. A capital letter is the main hue, lower-case letters are modifiers, and "sl" or "vsl" mean slightly or very slightly. So slpB is slightly purplish blue and bG is bluish green. The same idea of modifiers appears in coloured diamonds; see how modifiers and overtones work in coloured diamonds for the diamond version.

For most gems, the closer the hue is to the pure main colour, the higher the value. A modifier of about 10–15% purple in a blue sapphire is widely liked because it looks rich in warm light. A green modifier in the same sapphire usually lowers the price, and a strong one can turn it into a "teal" stone, which some people now choose for exactly that look.

What are tone and saturation in gemstones?

Tone is how light or dark the colour looks, ignoring the hue. On GIA's scale, 0 is colourless and 10 is black. Gem-quality stones almost always fall between about 2 (very very light) and 8 (dark). Saturation is the strength or purity of the colour, from 1 (greyish or brownish) to 6 (vivid). Cool hues such as blue, green and violet dull towards grey; warm hues such as red, orange and yellow dull towards brown.

Tone and saturation work together. A stone can be dark but greyish, which looks dull and steely, or light but vivid, which looks bright and fresh. Very dark tones also limit saturation, because the stone starts returning less light. That is why the sweet spot for most gems is medium to medium-dark, about 6–7 on the tone scale, with strong to vivid saturation.

In coloured diamonds, tone and saturation are combined into one intensity grade instead, running from Faint to Fancy Vivid. Our guide to the Faint to Vivid intensity scale shows how that compares.

What is the best colour for a sapphire, ruby, emerald and other gems?

Each gem has its own ideal, and the ideal is set by the market rather than by a rule book. Here is how gemologists generally describe the most sought-after colour in the stones most often set in Australian jewellery, along with the hardness (Mohs) and refractive index (RI, how strongly the stone bends light) that affect how the colour shows.

Australian sapphires from central Queensland and the New England region of New South Wales are a useful example. Many are naturally dark, tone 7–8, with an inky blue or greenish blue colour, so they are often cut shallower to lighten them. Australia is also known for parti sapphires, which show two or more colours, such as yellow and green, in one stone; these are judged on the pattern and balance of colour rather than a single hue.

Some gems have very wide colour ranges. Tourmaline comes in almost every hue, and the right choice is often personal; see tourmaline's huge colour range. Amethyst can lose colour with long exposure to strong light or heat, which our page on amethyst colour and why it can fade explains. If you are weighing up colour for a ring you will wear every day, start with choosing a coloured engagement stone.

Trade names are part of this too. "Pigeon's blood" (ruby), "royal blue" and "cornflower blue" (sapphire) and padparadscha (a pinkish orange to orangy pink sapphire) are used by some labs, each with their own criteria. They are descriptions of colour, not formal grades, and two labs may not agree on the same stone.

Why does a gemstone look a different colour in different light?

Colour is only as good as the light it is seen in. Gemologists judge colour under daylight-equivalent light, about 5,500–6,500 K (kelvin, a measure of how warm or cool the light is), against a neutral white or grey background. The same sapphire can look a little greener under cool LED lighting and a little more purple under warm household globes.

• Colour change: some gems change hue under different light. Alexandrite shifts from bluish green in daylight to purplish red under warm light; see alexandrite's green-to-red colour change. Colour-change sapphires typically move from blue to purple.
• Pleochroism: many gems show different colours when viewed along different crystal directions. Ruby and sapphire show two (dichroic); tanzanite shows three (trichroic). A good cutter orients the rough so the finest colour faces up.
• Colour zoning: uneven bands or patches of colour inside the stone, common in sapphire and amethyst. Zoning seen face-up lowers the grade; zoning only visible from the side often does not matter.
• Windowing: a washed-out, see-through patch in the centre where light leaks out of a shallow pavilion (the lower half of the stone).
• Extinction: dark, almost black areas, often at the ends of ovals or in very dark stones, where little light returns to the eye.

Because cut changes how much light comes back, the face-up colour is what is graded, not the colour of the rough. A stone with fine colour but a large window can look paler than its tone suggests, so always judge colour and cut together.

How is gemstone colour grading different from diamond colour grading?

White diamonds and coloured gems are graded for opposite reasons. In a colourless diamond, less colour is better, so labs grade it on a letter scale from D (colourless) to Z (light yellow or brown). In a coloured gem, the right colour, and enough of it, is what you want.

If you are comparing white diamonds, our guides to G colour diamonds, warmer J colour diamonds and G vs H for an engagement ring show how a single letter grade looks in a setting. For coloured diamonds, see how labs grade fancy colour diamonds, which uses a system much closer to the gemstone approach described here.

Are gemstone colours treated, and does it matter?

Many coloured gems are treated to improve their colour, and this is a normal, long-accepted part of the trade, as long as it is disclosed. The treatment does not change the grade of the colour you see, but it does change rarity and price.

• Heat treatment: very common in sapphire and ruby, used to deepen blue, remove silk or reduce purple. Almost all tanzanite is heated to bring out the violetish blue. Heat is stable and permanent.
• Lattice (beryllium) diffusion: a stronger process that adds colour from outside the stone, used on some orange, pink and yellow sapphires. It must be disclosed and sells for much less than heated-only stones.
• Oil or resin: used in emerald mainly to improve clarity, which also affects how evenly the colour appears.
• Irradiation: used on some topaz and tourmaline; in diamonds it is often combined with HPHT, covered in HPHT and irradiation treatments in diamonds.

Origin matters too. Lab-created (synthetic) sapphire, ruby, emerald and alexandrite have the same chemistry and hardness as the natural gems and are described with the same hue, tone and saturation terms. Their colour is often very even and saturated. They cost a small fraction of natural stones and must always be sold as lab-created or synthetic. Natural stones with fine, unheated colour are the rarest of all.

How does gemstone colour grading affect price?

For most coloured gems, colour sets the price more than anything else. Two 1 carat sapphires of the same size and clarity can differ several times over in price because one is a medium-dark vivid blue and the other is a light, slightly greyish blue. The same pattern drives coloured diamonds, as our page on what pushes fancy colour diamond prices up shows.

These are broad ranges only; prices vary with origin, treatment, cut quality and the market. Price also climbs steeply with size, because fine colour in a large crystal is rarer. In general, saturation is rewarded most, then tone, then hue: a vivid stone with a slight modifier usually outprices a pure-hue stone that looks greyish.

Does the metal setting change how a gemstone's colour looks?

Yes, more than many people expect. Metal reflects into the stone and frames its edges. White gold and platinum keep blues, purples and cool greens looking crisp. Yellow gold warms everything around it, which flatters rubies, orange and yellow sapphires and many emeralds, but can add a greenish cast to a pale blue stone such as aquamarine. Rose gold suits pinks and peaches, including morganite and padparadscha-coloured sapphires.

The setting style matters as well. An open setting lets light in from the sides and back, which can brighten a dark Australian sapphire. A full bezel with a closed back can deepen a pale stone but may make a dark one look darker still.

How do you judge gemstone colour yourself? Step by step

1. Use the right light. Look in daylight near a window, out of direct sun, or under a daylight-balanced lamp, away from coloured walls and clothing.
2. Look face-up. Judge the stone from above at normal viewing distance, the way it will be seen when worn.
3. Name the hue. Say the main colour and any modifier you notice, such as violetish blue or greenish blue.
4. Judge the tone. Against a white card, is it light, medium or dark? Aim for medium to medium-dark for most gems.
5. Judge the saturation. Look for grey in blues and greens or brown in reds and oranges. The less you see, the better.
6. Rock the stone. Tilt it slowly to check for a window, dark extinction areas and colour zoning across the face.
7. Check warm light too. Look under a warm globe to see how the colour shifts in the evening.
8. Read the report. Confirm the gem, whether it is natural or lab-created, and any treatments such as heat or diffusion.