Facet
One flat polished surface on a cut stone. Their number and angles decide how it returns light.
A standard round brilliant carries 57 or 58 facets, each cut to an angle calculated to bounce light back through the top.
The main regions are the crown above the widest point, the girdle at the widest point, and the pavilion below it.
Beads and cabochons are not faceted at all, which is why they glow rather than sparkle.
Cut
How a stone is shaped and faceted. The one thing that most affects how it looks, and the one buyers examine least.
Round Brilliant
The standard round cut with 57 or 58 facets, engineered to return the maximum light. The most common cut there is.
Cabochon
A smooth domed top with a flat base, no facets. The only way to show a cat's eye, star or opal fire.
Diamond
Crystallised carbon, the hardest natural material. Heera in Hindi, and graded on the four Cs.