The four Cs were designed for diamonds. Applied to coloured stones they mislead more than they help. Here is how each one actually works.
Carat
One carat is 200 milligrams. Density varies between gem varieties, so a one carat sapphire and a one carat aquamarine are different physical sizes — aquamarine is less dense, so a one carat aquamarine is visibly larger.
Millimetre dimensions matter more than carat weight when you are matching a stone to a setting. We list both on every product page.
Colour
Colour has three components, and all three matter:
- Hue — which colour it is. Blue, violet-blue, greenish-blue.
- Tone — how light or dark. Too light and the stone looks watery; too dark and it looks black.
- Saturation — how pure and intense. This is the one that drives value most.
A stone with excellent saturation and a slightly off hue will usually be worth more than one with a perfect hue and weak saturation.
Clarity
Coloured stones are judged by eye, not by loupe. The relevant question is not "how many inclusions are there" but "can I see anything that bothers me at arm's length".
Three things do matter:
- Surface reaching fractures — a durability risk in a ring.
- Large inclusions in the centre of the stone — visible and distracting.
- Inclusions that affect the colour — sometimes positively. Silk in sapphire can soften and spread colour.
Cut
The fourth C, and the one most often ignored. A well cut stone returns light to your eye. A badly cut one has a window — a see-through patch in the middle — or extinction, a dark dead zone.
Cut is where a knowledgeable buyer finds value, because it is invisible in a thumbnail photograph and heavily penalised in the price.
The short version
Buy colour first, cut second, clarity third, carat last. If a stone has beautiful colour and an honest cut, the rest is detail.