There are plenty of synthetic materials that would fall into the "cheap diamond equivalent" category. If we're only talking about the optical properties, cubic zirconia ($\ce{ZrO2}$) is probably the most familiar as it is often used in jewelry. It's index of refraction isn't quite as high and it's not as hard, but it's close enough that a fair amount of it is used to make jewelry. A more recent development is moissanite (either synthetic or natural $\ce{SiC}$), which is almost as hard as diamond and actually has a higher refractive index. Other synthetic materials with even higher indices like barium titanate and gallium phosphide exist, though generally their optical properties are used for things like lasers.
Of course, we have gotten quite good at making synthetic diamond itself. They are only used for a tiny fraction of diamond jewelry, though. It wouldn't surprise me to find a concerted effort to market natural diamonds as superior, given how lucrative the gem diamond market is, which might be preventing wider use for gems. Synthetic diamonds are widespread in industrial applications, however, where smaller diamonds are used to make tools and optical properties are largely irrelevant.