Coloured stones are not graded the way diamonds are. There is no universal scale. That makes buying harder, and it also means a well-informed buyer can do considerably better than the market average.
Start with colour
Colour is the majority of a coloured stone's value. Not size, not clarity — colour. A one carat sapphire with strong, even saturation will outshine a three carat stone that is grey or washed out, and it will hold its value better.
Look at the stone in daylight, not under a shop lamp. Most gemstone photography is done under controlled light that flatters the stone. Ask for a daylight photograph, or a video.
Then think about cut
A good cut puts the colour where you can see it. A bad cut leaves a window in the middle of the stone where light passes straight through, or a dark patch where it does not return.
Cut quality is also where you can find value. A beautifully coloured stone with a slightly asymmetrical outline costs noticeably less than a symmetrical one, and once it is set the asymmetry usually disappears.
Clarity matters less than you think
Unlike diamonds, coloured stones are judged with the naked eye. Inclusions that are invisible at arm's length do not affect how the stone looks, and in some varieties — Kashmir sapphire, for example — they are part of what produces the colour.
What matters is whether an inclusion affects durability. A surface reaching fracture in a ring stone is a problem. A tiny crystal deep inside a pendant stone is not.
Carat weight last
Weight is the easiest thing to sell and the least important thing to own. Large stones of poor colour are common and cheap. Small stones of excellent colour are rare and hold value.
Understand treatment
Heating is standard. Fracture filling and diffusion are not. Read our treatments and certification guide before you buy anything expensive.