COLOUR, CHOSEN FOR one.

Sapphire is the blue form of corundum, an aluminium oxide that crystallises deep in the crust under immense heat and pressure. Traces of iron and titanium give the stone its colour, from pale sky to deep midnight. The most celebrated origin is Sri Lanka, historically called Ceylon, whose stones show a vivid cornflower blue with a soft, velvet character prized for more than two thousand years.

At 9 on the Mohs scale, sapphire is second in hardness only to diamond among the classic gems, which makes it one of the few coloured stones truly suited to daily wear. Royal history agrees: the most famous engagement ring in the world, first worn by Princess Diana and now by the Princess of Wales, is a Ceylon sapphire.

When choosing, colour comes first: look for even saturation without inky zones or washed-out patches. Certification from a respected laboratory should state origin and any treatment. Gentle heating is an accepted, centuries-old practice; untreated stones of fine colour command a significant premium.

Ruby is corundum coloured red by chromium, the same mineral family as sapphire, and the rarest of the classic quartet. The finest stones show the colour dealers call pigeon blood: a pure, glowing red with a whisper of blue. Historic sources in Burma set the standard, while Madagascar and Mozambique produce the finest modern rubies.

Chromium gives ruby its fire, but it also fractures the crystal as it grows, so clean rubies above one carat are genuinely scarcer than diamonds of the same size. Prices at the top of the market reflect that rarity. Like sapphire, ruby wears beautifully every day.

Look for vivid colour and lively brilliance rather than perfect clarity. Certification should state origin and treatment: heat is standard and accepted, while glass-filled rubies have no place in fine jewellery.

Emerald is beryl coloured by chromium and vanadium, formed where colliding continents bring beryllium and chromium together: a geological coincidence so unlikely that fine emeralds remain among the rarest of gems. Colombia sets the world standard, while Brazil, where Bairro Jabour began, produces stones of extraordinary vivacity.

Almost every genuine emerald contains inclusions, known as the jardin, French for garden. They are accepted as part of the stone rather than a flaw, and they prove the stone is natural. Clarity enhancement with cedar oil is a standard, centuries-old practice; what matters is that it is disclosed.

Colour is everything: a vivid green with even saturation outranks a cleaner stone of paler colour. Emeralds favour protective settings, and they should never meet an ultrasonic cleaner. Warm water and a soft cloth are all they need.

Aquamarine is the sea-blue variety of beryl, the same family as emerald, coloured by traces of iron. Its name means seawater, and its colour runs from the palest sky to the deep, saturated blue the trade calls Santa Maria, after a mine in Minas Gerais. Brazil has been its great source for generations.

Where emerald is valued for its garden of inclusions, aquamarine often grows clean, so a fine stone is bright, transparent and usually eye-clean. At 7.5 to 8 on the Mohs scale, with good toughness, it is a sound choice for a ring worn every day.

Choose the purest blue you love, with little green. Gentle heating to remove a green tint is accepted and permanent. Emerald and oval cuts suit it beautifully, and like every coloured stone it should never meet an ultrasonic cleaner.

Fancy yellow diamonds owe their colour to nitrogen atoms scattered through the crystal lattice. Grades run from faint through fancy to fancy vivid, where the colour glows like captured sunlight. South Africa, and the famous Kimberley region in particular, has produced many of the great yellows.

In white diamonds, visible colour lowers the grade. In fancy yellows the rule inverts: saturation drives value, and the strongest, most even colours are the most sought after. As diamonds, they measure 10 on the Mohs scale, the hardest substance in nature.

Choose the richest, most even yellow your budget allows, ideally in a yellow gold setting that flatters the colour. A GIA report should state the colour grade and confirm natural colour.