Coloured gemstones

Why choose a coloured gemstone?

Coloured gemstones can make a custom engagement ring feel more personal, more distinctive, or more meaningful than a traditional diamond. Sapphires, rubies, and emeralds are the three most commonly chosen, but many other stones – including morganite, aquamarine, and spinel – are growing in popularity ⁽¹⁾. Colour choice is often deeply personal: some customers choose a stone for its appearance, others for its symbolism or because it is a birthstone.

How coloured gemstones are graded

The 4Cs used for diamonds (covered in Lesson 2.2.1) do not translate directly to coloured stones. For coloured gemstones, colour is the primary quality factor, not cut ⁽²⁾. Unlike diamonds, there is no single universal grading standard for coloured stones, and different laboratories may assess the same stone slightly differently ⁽³⁾. This means quality comparisons between coloured stones can be more subjective than they are for diamonds.

Colour in a gemstone is described using three components ⁽⁴⁾:

• Hue – the basic colour, such as red, blue, or green
• Tone – how light or dark that colour appears, on a scale from very light to very dark
• Saturation – how strong and pure the colour is, from dull or washed out to vivid and intense

The most valued stones tend to have a strong, vivid saturation and a medium tone. A stone that is too dark can appear almost black in certain lights; one that is too pale can look washed out ⁽⁴⁾.

Durability and the Mohs scale

Because an engagement ring is worn every day, hardness matters. The Mohs scale measures a mineral's resistance to scratching on a scale of 1 to 10 ⁽⁵⁾. The higher the number, the harder and more scratch-resistant the stone.

Key ratings relevant to engagement rings:

• Diamond: 10
• Sapphire and ruby: 9 – excellent for daily wear ⁽⁵⁾
• Emerald: 7.5–8 – reasonably hard, but emeralds often contain natural internal fractures that make them more prone to chipping under impact ⁽⁶⁾
• Morganite and aquamarine: 7.5–8 – suitable with care
• Stones rated 7 or below (such as amethyst or tanzanite) are generally considered too soft for daily wear in an engagement ring without extra protection from the setting ⁽⁵⁾

It is worth noting that hardness and toughness are not the same thing. A stone can be hard yet still crack if struck at a vulnerable point ⁽⁶⁾.

No treatment disclosure standard

Many coloured gemstones are routinely treated to improve their colour or clarity – heat treatment of sapphires is one common example. Unlike diamonds, where treatments must be declared, disclosure practices for coloured stones vary between sellers. A reputable jeweller should tell you whether a stone has been treated, and an independent gemological report from a recognised laboratory such as the GIA can confirm this ⁽³⁾.