Why Don’t All Gems Come in Any Cut?

I got the following question from two customers just this week: can I supply a matched pair of Merelani mint garnets, elongated pear shape.  The answer – and I don’t even have to look – is “no.”  (I actually did find one pair, so I was wrong, but you’ll see the astronomical price tag on my Etsy site!)
So why doesn’t any gem come in any cut?  The answer is really simple.  Gems are cut to maximize the rough.  Take, for instance, tourmaline rough, which is cylindrical and long.  That’s great for elongated pear shapes.  So is aqua and emerald sometimes (all three are beryl so the rough is very similar).  But most garnet rough is more roundish, pebble like.  That’s why it gets cut into ovals, some rounds, cushions, and anything else that accommodates that shape. 
Also, certain cuts bring out the beauty of the material, hence the famous “emerald” cut.  Emeralds are more sleepy than crisp, so an emerald cut, which has fewer facets, can bring out the color over the clarity.  Ceylon sapphires, by contrast, are usually cut into cushions and ovals, with step facets in the back.  Step cuts are bulkier but for Ceylon sapphires this is important because the color is sometimes just in the culet, and that cutting will create the illusion of an even tone when the gem is viewed from the front.  This explains the famous “zoning” that you can often see from the back (so it’s ok if you see it) but not the front (so it’s not ok if you see it).  Tourmalines that have watermelon coloring (pink inside, green at the rim) are often sliced or carved so that this effect is preserved.
Dravite Tourmaline Rough and Cut (Ovals are Best)

Diopside Rough
Some gems, i.e. amethyst, come in just about any shape.  That’s because amethyst is so cheap it can be bought by the kilo.  It then doesn’t matter how much you waste, most of the cost is in the cutting anyway.  The material is (nearly enough) free.  In other cases, i.e. tourmaline these days, the rough is very expensive.  And of course, when you buy and re-sell, your calculations are based on your cost.  If a customer wants you to do a special cut that wastes the material, your cost is still the same and so you will charge more for a smaller gem.
This also explains why despite what is commonly claimed, a retail customer does not necessarily lose out when she buys a stone that is cut to preserve weight.  Because if the seller marks up based on the cost of the rough, the buyer can get the most amount of weight – and hence value - out of her purchase.  Of course many sellers do not have a straightforward markup policy, they charge what the market will bear.   In that case the customer can lose out if she hasn’t looked at the prices of the gem she wants to buy (comparison shopping really pays in these cases).  That’s why it always pays for you as a customer to have some idea of the current market, of the most common cuts for gems, and for what is and isn’t possible based on the rough.  A few minutes of internet browsing of some of the common gem sites is usually enough to get a basic idea.
Burma Spinel Rough Crystal

Kyanite Rough, Just Shaved Around the Edges
Here are some of the most common cuts for popular gems:
Amethyst: cheap rough, any cut.
Alexandrite: elongated cushion, oval.  Round costs extra.
Aqua: any cut really, the rough is a nice size.
Citrine: cheap rough, any cut.
Emerald: Emerald cut and oval, some pears, rounds are at a premium.
Rhodolite garnet: any cut.
Ruby: Elongated cushions, ovals, then rounds (same as sapphire)
Sapphire: Elongated ushions, ovals, then rounds. Very few pears.  Step cuts for Ceylon.
Sphene: Ovals and Elongated cushions, then pears. Very few rounds.
Spinel: Cushion (square and elongated), oval, then round and pear.
Tanzanite: Lots of ovals and rounds, some cushions.  The rough is a bit bigger so more shapes are possible.
Tourmaline: Lots of longish cuts including emerald cut and pear shapes, not many rounds.
Tsavorite and Mint Garnet: Ovals, then all the other cuts.

A good rule of thumb is that oval is the cheapest and most common cut in most stones. Round is the most expensive.  
Watermelon Tourmaline Slices