[tkhoven]

It really depends on how loose your classification of "diamond" is and how large you want. Cubic zirconia is quite popular with hobbyist and some professional faceters and is quite readily available in a range of colours (try searching for cubic zirconia rough) but it's more difficult to manufacture the larger you get. Moissanite is much closer to natural diamond in both hardness and refractive index but it still seems to be fairly inexpensive uncut. Not too familiar with other diamond synthetics but the range seems to be growing.