Sapphire

Overview

Sapphire is gem-quality corundum, crystalline aluminium oxide. Red corundum is called ruby; every other colour is sapphire. At 9 on the Mohs scale, second only to diamond among popular gems, and with no cleavage, sapphire is ideal for jewellery worn every day.

Colour

Blue sapphire owes its colour to iron and titanium. The most valued stones are a strong, velvety blue to violetish blue of medium to medium-dark tone. Sapphires in other colours are called fancy sapphires: pink, purple, yellow, orange, green and colourless. Padparadscha, a delicate pinkish orange, is among the rarest. Some sapphires change colour between daylight and warm light, and some show a six-rayed star (asterism) when cut as cabochons.

Key properties

Where sapphire comes from

Sri Lanka and Madagascar are leading sources of fine blue and fancy sapphires. Myanmar, Thailand, Cambodia, Australia, Montana in the USA, Tanzania (including the Umba valley and Tunduru), Kenya and Nigeria also produce gem material. Kashmir sapphires, mined mainly in the late 19th century, remain the most celebrated.

Treatments

• Heating is the most common treatment and is widely accepted. It improves colour and dissolves fine "silk" inclusions.
• Diffusion adds colour from outside the stone: beryllium diffusion can create orange, yellow and padparadscha-like colours; titanium diffusion adds a shallow surface blue.
• Lead-glass or flux filling hides fractures and greatly reduces value and durability.

All treatments must be disclosed. Unheated sapphires of good colour command a significant premium, and their status should be confirmed by a laboratory report.

Buying tips

• Colour first: vivid and even, not too dark under indoor light.
• Clarity: fine silk is normal and can soften the look attractively; avoid inclusions that reach the surface.
• Cut: sapphires are often cut deep to save weight. A stone that looks small for its weight may be over-deep.
• Get a report for significant purchases, stating treatment and, if relevant, origin.

Care and cleaning

Clean with warm soapy water and a soft brush. Ultrasonic and steam cleaning are usually safe for untreated and heat-treated sapphires, but never for fracture-filled or glass-filled stones. Sapphire can scratch other gems, so store it separately.