Carats have been the standard for weighing gems since ancient times. “The name is derived from the see kuara of the African Coraltree or from the kernel (Greek keratiton) of the Carob bean.”  One carat is a unit of mass equivalent to 200 micrograms, 0.2 grams, 0.007 ounces, or about the same as a standard paper clip.
Both cut and stone density affect the total carat weight of a stone. For example, I sampled five 5mm round sapphires and the carat weight varied from 0.615cts to 0.835cts as a result of differences in how each stone was cut. To compare to other gemstones, I found a 5mm round andalusite weighing in at 0.45cts, kyanite at 0.6cts, and a brilliant cut diamond at 0.5cts.
One thing to note as you are looking at gemstones is that larger stones are more rare and will likely cost more per carat than the more easily obtainable, small stones.
 Gemstones of the World: Newly Revised & Expanded Fourth Edition by Walter Schumann, p30