How to buy a coloured gemstone
Buying a coloured stone is different from buying a diamond. There is no single grading scale that every laboratory and dealer uses, and two stones with the same name and weight can differ in price by a factor of ten. This guide explains what we look at when we buy a stone, so you can judge our listings, and anyone else’s, with more confidence.
The four Cs, adapted for coloured stones
The familiar four Cs of colour, clarity, cut and carat weight apply to coloured stones too, but their importance is very different.
- Colour is by far the most important factor, often accounting for most of a stone’s value. Dealers look at three things: hue (the basic colour, such as red or bluish green), tone (how light or dark it is) and saturation (how strong or pure the colour is). The most valuable stones usually have a pure hue, a medium to medium-dark tone and strong saturation. A stone that is too dark looks black in dim light, and one that is too light looks washed out.
- Clarity expectations depend on the species. Aquamarine, topaz and citrine are normally clean, so visible inclusions reduce their value noticeably. Emerald and red tourmaline almost always contain inclusions, and a clean emerald is exceptionally rare. Judge clarity against what is normal for the species rather than against diamonds.
- Cut in coloured stones is about showing the colour well rather than following ideal proportions. A good cut has an even outline, reasonable symmetry and no large window, a washed-out area in the centre where light passes straight through. Many stones are cut deep or lopsided to save weight from the rough; we try to avoid those.
- Carat weight measures weight, not size. A one-carat ruby is smaller than a one-carat aquamarine because ruby is denser. Price per carat rises with size, as large fine crystals are rarer, so check the dimensions in millimetres as well as the weight.
Heat treatment explained
Heating is the oldest and most widely accepted gemstone treatment. Gem traders in Sri Lanka and Southeast Asia have used it for centuries. Controlled heat can dissolve fine silk inclusions, remove unwanted brown or green tones and deepen or even out colour. The results are permanent and do not need special care.
Most rubies, sapphires, tanzanites, blue zircons and aquamarines on the market are heated, and the trade accepts this as standard. Unheated stones of fine colour are much rarer and command a premium, often 50 to 100 percent or more for fine ruby and sapphire. That premium only makes sense if the stone really is unheated, which is why we recommend an independent laboratory report for any valuable unheated stone.
Other treatments are also common and must be disclosed. Emeralds are routinely filled with oil or resin to reduce the visibility of fissures; minor oil is the mildest grade. Blue topaz is irradiated. Some treatments, such as lead-glass filling in ruby or beryllium diffusion in sapphire, change a stone far more and greatly reduce its value. We do not sell those stones without stating it clearly.
What “eye clean” means
A stone is described as eye clean when no inclusions can be seen face-up with the naked eye at a normal viewing distance of about 25 to 30 centimetres. Inclusions may still be visible under a 10x loupe. Loupe clean means no inclusions are visible even at 10x magnification. Slightly included means small inclusions can be seen with the eye if you look closely.
Laboratories group coloured stones into three clarity types. Type I stones, such as aquamarine and topaz, usually grow clean. Type II stones, such as ruby, sapphire and garnet, usually have some inclusions. Type III stones, such as emerald and red tourmaline, are almost always included. An eye-clean Type III stone is a rare find.
Origin premiums
For some gems, the country of origin has a strong influence on price. Burmese rubies, Kashmir sapphires and Colombian emeralds have historic reputations and sell at a premium over stones of similar appearance from other sources. Some of that premium reflects real differences in colour, such as the strong fluorescence of Burmese ruby or the velvety look of Kashmir sapphire. Some of it reflects scarcity and history.
Origin can only be determined with confidence by a laboratory with advanced instruments. If you are paying for a named origin, make sure it is backed by a report from a respected laboratory. A fine stone from a less famous source can offer better value if origin matters less to you than appearance.
How lots and parcels are graded
Lots are groups of stones sold together, and their value depends on how well they match. When we assemble a lot, we compare the stones side by side in daylight and under warm light, checking hue, tone and saturation, and remove any stone that stands out. Calibrated lots are measured on a millimetre gauge so that every stone fits a standard setting. We describe a lot’s clarity by its overall grade and mention any stones that fall below it.
Parcels are graded in the same way, but on a larger scale: we sort the stones by size tolerance and colour band so that a buyer ordering twenty pieces today will receive stones that match the twenty ordered next month.
Caring for your stones
- Store stones separately in gem papers or soft pouches. Harder stones such as sapphire will scratch softer ones.
- Clean most faceted stones with warm water, mild soap and a soft brush. Rinse and dry with a lint-free cloth.
- Do not put emeralds, opals, tanzanites, moonstones or any fissured or treated stone in an ultrasonic or steam cleaner.
- Keep opal away from oils, lotions and prolonged soaking, and avoid sudden temperature changes for peridot, tanzanite and opal.
- Long exposure to strong sunlight can fade amethyst and some pink stones over the years.
- Tell your jeweller what the stone is and how it has been treated before setting, re-sizing or soldering near it.
If you have questions about a specific stone, write to us. We would rather spend ten minutes explaining a stone before you buy it than have you disappointed after it arrives.