The 4Cs, in plain English

Cut is how well a diamond's facets return light. It affects sparkle more than anything else. Grades run from Ideal or Excellent down to Fair. A well-cut stone outshines a bigger, badly cut one.

Graded from D (icy white) to Z (tinted). D to F are colourless. G to J are near-colourless, look white once set and cost far less.

Measures tiny internal marks (inclusions), from Flawless (FL) to Included (I3). VS1 to SI1 stones are usually eye-clean: you can't see anything without magnification.

Carat is weight, not size. Price jumps at round numbers (1.0ct, 2.0ct), so 0.9ct can cost far less than 1.0ct while looking almost identical.

Lab-grown diamonds have the same chemistry and look as mined diamonds, and the same labs grade them. They usually cost 70 to 90% less for the same grades.

Natural diamonds hold more of their resale value. Which to choose comes down to budget and personal preference.

We group diamonds by type, shape, carat range, colour, clarity and cut, and take the median price of each group. That median is the fair price. A diamond below it costs less than most stones with the same grades.

It’s an estimate, not a valuation. Use it to judge a quote, not to insure a ring.