Tanzanite Gemstones – Properties, Origins, Treatments and Buying Guide

Tanzanite is one of the rarest and most captivating gemstones in the world, prized for its intense blue to violet hues and exceptional brilliance. Discovered only in the 20th century, tanzanite is found in a single location on Earth, making it far rarer than diamond.

Renowned for its striking colour shift and luxurious appearance, tanzanite has become one of the most desirable coloured gemstones in fine jewellery. In this guide, we explore tanzanite from a professional gemmological and commercial perspective, covering its physical properties, crystal structure, treatments, origins and how to identify fine-quality stones.

Tanzanite is the blue to violet variety of the mineral zoisite. It was first discovered in 1967 in Tanzania and quickly became one of the most sought-after gemstones in the world.

Its unique colour is caused by trace amounts of vanadium, and its famous blue-violet appearance is enhanced through heat treatment. Tanzanite is valued for its rarity, vibrant colour and strong pleochroism, meaning it can show different colours depending on viewing angle.

Tanzanite has the chemical formula:

Ca₂Al₃(SiO₄)₃(OH)

It is composed primarily of:

Calcium
Aluminium
Silicon
Oxygen
Trace vanadium (responsible for colour)

The presence of vanadium gives tanzanite its distinctive blue, violet and purplish hues, which are intensified through heat treatment.

Tanzanite belongs to the zoisite mineral group, part of the silicate family.

Its crystal system is orthorhombic, which contributes to its strong pleochroism — a defining feature where different colours appear when viewed from different angles.

Tanzanite is prized for its vivid colour, brilliance and optical character.

Lustre
Tanzanite has a vitreous to slightly oily lustre, giving it excellent visual depth.

Transparency
Fine-quality tanzanite is transparent with excellent clarity.

Refractive index
1.69 – 1.70, giving strong brilliance and fire.

Specific gravity
3.10 – 3.38, typical of zoisite minerals.

Tanzanite displays a unique range of colours:

Deep royal blue
Violet-blue
Bluish purple
Lavender tones

The most valuable stones exhibit a rich, saturated blue with violet undertones.

Tanzanite has a Mohs hardness of 6 to 6.5.

While suitable for jewellery, it is softer than sapphire or diamond and should be worn with care, especially in rings.

Almost all tanzanite is heat treated to enhance its blue-violet colour.

This treatment is permanent, stable and fully accepted within the gemstone industry.
No coatings or fracture fillings should be present in fine-quality tanzanite.

Avoid ultrasonic and steam cleaners.
Use warm water, mild soap and a soft cloth.
Store separately from harder gemstones to prevent scratching.

Tanzanite is found in only one place on Earth:

Merelani Hills, Northern Tanzania

This extreme geographic limitation makes tanzanite one of the rarest commercial gemstones in the world.