Ruby

Ruby is red corundum, the same mineral as sapphire, coloured by traces of chromium. The chromium that gives ruby its colour also makes it glow: under the ultraviolet in daylight, many rubies fluoresce red, which is part of why the finest look as if lit from within.

It is the birthstone for July, and in Sanskrit it was called ratnaraj, the king of precious stones.

The most valued rubies are a pure, vivid red to slightly purplish red, of medium to medium-dark tone. The trade term pigeon’s blood describes the most famous of these: a saturated, glowing red associated with the historic mines of Mogok in Myanmar. Stones that lean orange, brown or pink are generally valued less.

Today Mozambique and Madagascar are important sources of fine rubies, alongside Myanmar and the Thai and Cambodian deposits that produce darker, iron-rich stones.

Because ruby and sapphire are the same mineral, the line between a light ruby and a pink sapphire is a matter of colour saturation and tone, and laboratories do not always draw it in the same place. If the name matters to you, ask what the report calls the stone.

Most rubies on the market have been heated to improve colour and clarity. Heating is permanent, centuries old and accepted across the trade. Unheated rubies of fine colour are rare and command a significant premium.

Some lower-grade rubies are filled with lead glass to hide extensive fractures. These composite stones are far less durable and far less valuable, and they can be damaged by heat and household chemicals. They have no place in a fine engagement ring. A reputable laboratory report states origin and any treatment: read it.

Rubies almost always contain inclusions. Fine needles called silk are common and can soften the light pleasantly. Look for a stone that is bright and lively rather than flawless.

At 9 on the Mohs scale, ruby is hard and tough enough for everyday wear in any setting. It glows beside white diamonds, in platinum or in yellow gold, and makes a striking centre for a halo or a three stone ring.

Clean it with warm water, a drop of neutral soap and a soft brush, then rinse and dry with a soft cloth. Never put a gemstone in an ultrasonic cleaner or a steamer.