Know what you are buying

Judging a coloured stone

Colour decides most of the value. Gemmologists describe it in three parts: hue (the colour itself, such as green or slightly bluish green), tone (how light or dark) and saturation (how vivid). The most valued stones have a pure hue, a medium to medium-dark tone and strong saturation: dark enough to look rich, light enough to stay bright.

Clarity is judged against what is normal for the gem. Some gems, like aquamarine and topaz, usually grow clean, so visible inclusions lower their value sharply. Rubies and sapphires usually contain some inclusions. Emeralds almost always do, and a few are accepted, even admired as the emerald's "jardin". "Eye clean" means no inclusions are visible to the unaided eye at a normal viewing distance.

Cut is about light, not only shape. A well-cut stone returns light evenly across its face with few dark or washed-out areas ("windows"). Coloured stones are often cut to keep weight, so proportions vary more than in diamonds.

Carat is weight: one carat is 0.2 grams. Different gems have different densities, so a one-carat ruby looks smaller than a one-carat emerald. Price per carat usually rises with size, because large fine stones are rarer. That is why our listings show both the price and the price per carat.

Treatments explained

Many gemstones are treated to improve colour or clarity. Treatment is not wrong; hiding it is. A treated stone is worth less than an untreated stone of the same appearance, which is why every listing states it.

• Heating: the most common treatment for rubies and sapphires, improving colour and clarity. It is stable and permanent. Unheated stones of fine quality command a premium.
• Oiling or resin: fills surface-reaching fissures in emeralds with colourless oil or resin so they show less. Minor oiling is the industry norm; significant amounts reduce value and need gentle care.
• Glass or lead-glass filling: fills large fractures, mostly in low-grade rubies. These stones are fragile and worth a fraction of an untreated ruby.
• Diffusion: adds colour by heating a stone with elements such as beryllium. It must always be disclosed.
• Irradiation: produces colours such as the blue of most blue topaz. It is stable once released for sale.
• Dyeing: adds colour to porous stones such as agate and chalcedony. Our aqeeq is natural colour, never dyed.
• Stabilisation: impregnates porous stones such as turquoise with resin to harden them. Our turquoise is natural, not stabilised.

Origin and origin reports

For some gems, origin moves the price: Kashmir sapphire, Burmese ruby, Colombian or Swat emerald. A laboratory forms an origin opinion from a stone's inclusions, its trace-element chemistry and its spectroscopy, compared with reference stones from known deposits. It is an expert opinion, not a certainty, and laboratories occasionally disagree. Where we bought a stone at the mine we say so, and where origin rests on a laboratory opinion we name the laboratory.

Natural, laboratory-grown, imitation

A natural stone formed in the earth. A laboratory-grown (synthetic) stone has essentially the same chemistry, crystal structure and optical properties, but was grown in a laboratory in weeks or months; it must be named "laboratory-grown", "laboratory-created" or "synthetic", as clearly as the gem's name. An imitation or simulant only looks similar (glass, cubic zirconia for diamond) and is a different material altogether. Telling natural from laboratory-grown often needs a gemmological laboratory, which is one reason certificates matter.

Diamonds: the 4Cs

Diamonds are graded on international scales developed by GIA. Colour runs from D (colourless) to Z (light yellow or brown). Clarity runs from Flawless through VVS and VS to SI and I (included). Cut grades (Excellent to Poor) describe how well a round diamond handles light. Carat is weight. Many diamonds also show fluorescence, usually blue, under ultraviolet light; strong fluorescence can make a few stones look hazy.

Mineral specimens

Specimens are collected for their crystals, not cut. Collectors describe size in classes: thumbnail (fits a box about 2.5 cm across), miniature (up to about 5 cm), small cabinet (about 5 to 10 cm) and cabinet (larger). The full locality (mine, district, country) matters as much as the species, and any damage or repair must be disclosed. Some minerals glow under ultraviolet light (fluorescence), which is shown in a UV photograph.