Sapphire Gemstones – Properties, Origins, Treatments and Buying Guide

Sapphire is a precious gemstone and a variety of corundum, most famously blue but also found in other colours. Renowned for its durability and brilliance, sapphire has been treasured for centuries by royalty, collectors, and jewellery enthusiasts.

In this guide we examine sapphire from a professional gemmological perspective, covering its properties, crystal structure, treatments, origins, and tips for identifying high-quality stones.

What Is Sapphire?

Sapphire is the gem-quality form of corundum, an aluminium oxide mineral. Trace elements such as iron and titanium produce blue colours, while chromium, vanadium, and other elements create pink, yellow, and other hues.

Sapphire is formed in metamorphic and igneous rocks and is known for its exceptional hardness and brilliance.

Sapphire has the chemical formula:

Al₂O₃

It is composed primarily of:

• Aluminium
• Oxygen
• Trace elements: iron, titanium, chromium, vanadium (colour variation)

The presence and combination of trace elements determines the sapphire’s colour and intensity.

Sapphire belongs to the oxide mineral group and crystallises in the trigonal crystal system. This structure contributes to its high durability, characteristic hexagonal prismatic crystals, and optical properties.

Sapphire is prized for its combination of brilliance, durability, and refractive properties that make it highly sought-after for jewellery.

Lustre
Sapphire exhibits a vitreous (glassy) lustre, creating a bright, reflective surface on well-cut stones.

Transparency
Gem-quality sapphire is transparent. Lower-grade material is used for cabochons or ornamental carving.

Refractive Index
1.76 – 1.77. Sapphire has strong brilliance and minimal double refraction, enhancing clarity and sparkle.

Specific Gravity
3.95 – 4.03. Sapphire is heavier than most gemstones, contributing to its solid feel and quality perception.

Sapphire occurs in multiple colours depending on trace elements:

• Blue (classic)
• Pink
• Yellow
• Green
• Purple
• Colourless (white sapphire)

The most valued sapphires are vivid, saturated blue with strong clarity and brilliance.

Sapphire has a Mohs hardness of 9, making it extremely durable.

It also has:

• Excellent cleavage resistance
• High toughness

This makes sapphire ideal for all types of jewellery, including rings worn daily.

Most sapphires are heat-treated to enhance colour and clarity.

Common treatments:

• Heat treatment (standard)
• Surface diffusion (rare)

Untreated sapphires are rare and significantly more valuable.

Sapphire is durable but still benefits from proper care:

Avoid:

• Harsh chemicals
• Extreme temperature changes

Safe cleaning method:

• Warm water
• Mild soap
• Soft brush or cloth

Important sources of sapphire include:

• Kashmir, India – finest vivid blue material
• Myanmar (Burma)
• Sri Lanka
• Madagascar
• Australia
• Thailand
• Montana, USA