Two stones can look identical to a casual eye and differ tenfold in price. The difference comes down to four value factors professionals weigh: colour, clarity, carat and cut.
Colour
In coloured stones, colour is the most important factor — hue, tone and saturation together. A stone with vivid, well-balanced saturation commands a premium over a pale or overly dark one of the same variety. In diamonds, the scale runs from colourless towards tinted, and the top grades carry steep premiums.
Clarity
Clarity is the presence of inclusions and surface features. Each gem variety has its own clarity expectations — some types almost always form with inclusions, so the standard differs by stone. What matters is how inclusions affect beauty and durability, not their mere existence.
Carat and cut
Carat is weight, and price rises non-linearly: a 2-carat stone of equal quality costs far more than two 1-carat stones, because large clean rough is rarer. Cut covers both shape and quality of fashioning — proportions that return light to the eye in faceted stones, and skilled shaping in cabochons. A poor cut drains value from fine rough.
Learning to judge
Practical colour description, clarity types, cutting styles and carat interpretation are taught hands-on with real stones in the gem curriculum at JDMIS — see the gem and jewellery trade programme for the full pathway.
Frequently asked questions
What are the four value factors for gemstones?
Colour, clarity, carat and cut. In coloured stones colour is the most important factor; clarity standards differ by variety; carat price rises non-linearly because large clean rough is rarer; and cut quality decides whether light returns to the eye.
Why does a 2-carat stone cost more than two 1-carat stones?
Because large clean rough is rarer. Price rises non-linearly with weight, so a 2-carat stone of equal quality costs far more than two 1-carat stones combined.
Explore further: play Name That Gem, a free gem-identification game, or trace gems to their sources on the Gem Globe.
Going deeper: these are bench skills you can learn properly - the gemmology foundation course and the gem identification methods certificate teach identification with real stones and instruments.