Some are natural and some have been laboratory treated. Those that have been treated are worth less than natural colored stones. There are 100 points in a carat, a unit of weight equal to 200 milligrams. A half-carat stone would therefore weigh 50 points. The important thing is to get the weight of the diamond in points, not just the fraction.
Sellers have their own tolerance ranges for the fractions they use.
A carat can be 94 points to 100 points depending on who is selling it. A 94 point stone is not worth what a true carat or 100 point stone is. Larger diamonds are rarer than smaller diamonds, so a larger stone costs more per carat. Therefore, two half-carat diamonds are not worth what a one carat stone of equal quality is worth. A diamond gets it's brilliance from the way it is cut.