Gems are sparkly. But they don’t necessarily look all that flash when they come up out of the ground. The reason they sparkle so is due to the gemcutter, who cuts and polishes a gem to give it angles and facets so that light entering the gem is bounced around and back out at the viewer.
A cut gem has several components: the top surface is the table, which generally tapers outwards through the crown to the girdle. Then the gem is tapered inwards again through the depth of the pavilion to the end point, the culet.
There are all sorts of different cuts, with varying degrees of sparkle. There is the Round or Brilliant cut, which is indeed both round and brilliant. It generally has 58 facets! That’s quite a lot. The Oval, Pear-shaped and Heart-shaped cuts appear as you might expect; the Oval is apparently popular with “women with small hands or short fingers” as the shape helps to elongate the appearance of the hand. I suppose stubby-fingered blokes do not wear gems.
The Marquise cut is like an oval with pointy ends, apparently inspired by the smile of the Marquise du Pompadour and commissioned by Louise XIV (the Sun King). It is also called the Navette cut, which is the French word for “little boat” (because it resembles the hull of a boat). I’m not sure I believe that thing about the Marquise’s smile, it seems a bit weird.
The Emerald cut is a rectangle with blunted corners and straight facets. It is also called a step facet because the concentric facets look like steps. The Radiant cut is similar only it has a rounded Brilliant bottom. The Princess, too, has a Brilliant bottom but from the top appears square.
There is also the Baguette, Cushion, Flander, Obus, Pendeloque, Trillion, Magna, Lozenge and many more. The gem cutter will chose a style that makes the best use of the stone, so as much weight as possible is retained, but also keeping in mind what styles are popular.
Stones may be recut – the famous and famously cursed Koh-I-Noor diamond (part of the British Crown Jewels) was recut in 1852 to improve its sparkle, under supervision of Prince Albert (consort to Queen Victoria), after general disappointment in its appearance when it was shown at the World Fair of 1851. The cutting took 38 days and the stone was reduced from 186 1/16 carats (37.21 g) to 105.602 carats (21.61 g) – a weight reduction of 42%. Prince Albert dropped about £8,000 on the recutting of the Koh-I-Noor and still wasn’t happy with the result.