Figuring Rough Cost
The basic rule of thumb for figuring rough cost is that there are five carats in a gram and if you figure an average finished yield of 30%. Then for every gram the rough weighs you will get 1.5 carats per gram finished cut stone. If I am not sure how the rough will yield I figure 20% to be on the safe side. Which is 1 carat per gram of rough. Remember the yield will go up as the stones get larger, and some materials like Tourmaline in an emerald cut will yield much better than 30%. So these are just estimates.
For example, you are cutting a piece of Tourmaline that you paid $8 per carat for rough, and the rough/stone weighs 10 cts, it cost you $80.00 for the rough. Your finished stone will be 3 carats (30% yield, if you do not have any problems). So $80.00 cost / 3 carats = $26.66 per carat, this is your finished cost per carat.
A down and dirty, quick easy way to figure yields (on the conservative side) is to figure the yield at 1 finished carat per gram (20% yield). This method is easy to use because you just figure 1 carat per gram of rough weight. If a piece of rough costs you $40.00 per gram rough, then your finished cost is $40.00 carat.