How to choose a coloured gemstone

Start with colour

For coloured gemstones, colour accounts for most of the value. Gemmologists describe it in three parts:

• Hue: the basic colour, such as blue, or a secondary hue like greenish blue.
• Tone: how light or dark the colour is. The best stones are usually medium to medium-dark: dark enough to look rich, light enough to stay bright.
• Saturation: the strength of the colour. Vivid stones are rare; greyish or brownish ones are less valuable.

Look at stones in daylight or a daylight-balanced lamp, and also under warm indoor light: some gems change noticeably between the two. Even colour across the stone matters too; visible bands or zones lower value.

Clarity: what is normal for each gem

Coloured stones are judged against what is typical for their species, not against diamond.

• Usually clean: aquamarine, topaz, citrine, blue and green tourmaline. Visible inclusions should lower the price.
• Usually slightly included: sapphire, garnet, amethyst, chrysoprase.
• Usually included: emerald and red or pink tourmaline. Inclusions are accepted as long as they do not threaten durability or spoil the beauty of the stone.

Inclusions reaching the surface, especially near corners and the girdle, can weaken a stone. Treat those with more caution than inclusions deep inside.

Cut: brilliance and shape

A good cut shows colour evenly and returns light to the eye. Watch for:

• Windowing: a washed-out patch in the centre where light passes straight through a shallow pavilion.
• Extinction: dark areas that stay dark as you move the stone.
• Symmetry and finish: even facets and a good polish.

Many coloured stones are cut to save weight from valuable rough, so expect some variation. Cabochons (smooth, domed stones) suit included or opaque materials and show effects such as chatoyancy and asterism.

Carat weight and size

A carat is 0.2 grams. Gems differ in density, so stones of the same weight can differ in size: a one-carat sapphire looks smaller than a one-carat emerald. Compare dimensions in millimetres as well as weight. Prices per carat usually jump at popular weights (1, 2, 3 and 5 carats), so a stone just below them can be good value.

Treatments

Many gems are treated to improve colour or clarity: heating sapphire and aquamarine, oiling emerald, irradiating blue topaz. Treatment is not wrong, but it must be disclosed, and it affects value and care. Untreated stones of fine quality are rarer and cost more. Ask the seller in writing, and for important stones request a laboratory report.

Origin

Some origins carry a premium, such as Kashmir or Sri Lankan sapphire, Colombian emerald and Brazilian Paraíba tourmaline. Origin can only be determined reliably by a recognised laboratory. Buy the stone for its beauty first; treat origin as a bonus.

Durability for everyday wear

Think about how the piece will be worn. Rings take knocks; earrings and pendants do not. The Mohs scale describes scratch resistance (dust contains quartz, at 7, so softer gems dull over time), but toughness and cleavage matter too: topaz is hard but can split along its cleavage. For daily rings, choose sapphire, or protect softer stones with a bezel or a guarding setting.

Certificates and paperwork

For valuable stones, a report from an independent laboratory (for example GIA, SSEF, Gübelin, AGL or GRS) confirms the species, whether it is natural, any treatments and sometimes origin. Keep it with your purchase receipt for insurance.

A checklist before you buy

1. Is the colour vivid and even, in both daylight and indoor light?
2. Is the clarity typical (or better) for the species?
3. Is the cut bright, without a window or large dark areas?
4. Have all treatments been disclosed in writing?
5. Does the stone suit how the jewellery will be worn?
6. Is there a laboratory report for a significant purchase?

If you are unsure, ask us. We are happy to send extra photographs or a video of any stone in daylight before you decide.