Cut
How a stone is shaped and faceted. The one thing that most affects how it looks, and the one buyers examine least.
Cut covers two separate ideas: the outline, such as round or pear, and the quality of the proportions and facet placement.
Proportions decide how much light returns to the eye. A poorly proportioned stone leaks light through its base and looks dull whatever its colour or clarity.
Cut is also the only one of the four Cs a human controls. Colour, clarity and carat come out of the ground.
When you are buying
Given a fixed budget, take a smaller, better-cut stone over a larger dull one. Size is what a certificate records; cut is what people actually see.
Round Brilliant
The standard round cut with 57 or 58 facets, engineered to return the maximum light. The most common cut there is.
Facet
One flat polished surface on a cut stone. Their number and angles decide how it returns light.
Diamond
Crystallised carbon, the hardest natural material. Heera in Hindi, and graded on the four Cs.
Carat
A unit of gemstone weight, equal to 0.2 grams. Nothing to do with karat, which measures gold purity.