Gemstone Basics for Buyers: Weight, Clarity and Treatment
Three ideas explain most of a stone's price. Understanding them is enough to ask a seller the right questions.
Weight
A gemstone is weighed in carats; one carat is 200 milligrams. Price per carat rises with size, so two half-carat stones cost less than a single one-carat stone of the same quality.
Clarity
Most stones contain small internal features. For diamonds there is a formal scale; see diamond clarity. For coloured stones, what matters is whether marks are visible to the eye.
Treatment
Many stones are heated or otherwise treated to improve colour. It is common and acceptable when disclosed. An untreated stone of the same appearance costs more.
Questions to ask
- Is it natural, synthetic or imitation?
- Has it been treated, and how?
- Is there a report from an independent laboratory?
Buy the stone you like
Grades help compare prices. They do not tell you whether you will enjoy wearing it.
Reviewed 2026-10-14