Ruby

Ruby is corundum, the same mineral as sapphire, coloured red by chromium. The more chromium, the stronger the red and the more the stone glows. In Sanskrit it was ratnaraj, the king of gems, and it can still command the highest price per carat of any coloured stone.

For centuries the most famous rubies came from Myanmar. In 2009 a major deposit was found near Montepuez in northern Mozambique, and within a few years the trade was calling it the world's largest supplier.

Where the name comes from

The name comes from the Latin ruber, red.

Beliefs and legends

• A fire inside. People believed ruby's glow came from a flame burning inside the stone, hot enough to boil water.
• Invincible in battle. Burmese warriors believed rubies made them invincible, and some even set rubies under their own skin.
• Peace with enemies. In ancient India, a fine ruby was said to let its owner live at peace with their enemies.
• Health, wealth and love. Medieval Europeans wore rubies to bring health, wealth, wisdom and success in love.

The colours

• Pure red to slightly purplish red: the finest colour.
• Orange or purple tints: lower the value in most markets.
• Too light: then it may be called pink sapphire rather than ruby.
• "Pigeon's blood": a trade term for a vivid red with a soft red glow, historically tied to Burmese rubies. It describes a look and a reputation, not a laboratory grade.
• Star ruby: fine needles of rutile, called silk, can make a cabochon show a six-rayed star.

Mozambique's rubies

Rubies were found near Montepuez, about 150 kilometres west of the port of Pemba, in May 2009. A mining concession was granted in 2012, and by 2014 the deposit had produced around eight million carats of ruby and sapphire. Their colour sits between the glowing reds of Myanmar and the darker reds of Thailand, and the finest come from a part of the deposit called Maninge Nice.

Treatments, honestly

Most rubies are heated to improve their colour and clarity, and heated rubies are as durable as untreated ones. Some low-grade rubies are filled with lead glass to hide fractures. That treatment is much less durable, needs special care, and must always be disclosed. Some dark stones have colour added by beryllium diffusion. Untreated rubies exist, including some from Mozambique, and command a premium. We tell you what we know about each stone before you buy it.

How to judge a ruby

• Colour is everything: a pure, vivid red of medium tone.
• Clarity: a ruby with no inclusions is very rare; fractures reach the surface and weaken the stone.
• Cut: most rubies are cut as ovals and cushions. A shallow stone can show a pale "window" in the centre.
• Size: fine rubies over one carat are very rare, and the price per carat climbs steeply.
• Ask about treatment in writing: unheated, heated or glass-filled.

Caring for your ruby

Ruby is 9 on the Mohs scale, tough and without cleavage, so it suits everyday rings. Untreated and heated rubies can be cleaned in warm soapy water, an ultrasonic or steam. Glass-filled rubies need only a damp cloth: even lemon juice can damage the glass.

At a glance

Mineral	Corundum (aluminium oxide), red variety
Hardness	9 Mohs; tough, no cleavage
Colours	Red to purplish red; pink is pink sapphire
From	Mozambique (Montepuez), Tanzania, Madagascar; also Myanmar
Treatments	Most heated; some glass-filled or diffused. Disclosed per stone
Care	Untreated/heated: soapy water, ultrasonic, steam. Glass-filled: damp cloth only