Diamonds are a fascinating and unique material. So far, though, manufacturing large diamonds on a large scale isn't very economical, but there actually are a few processes that do make small, commercial synthetic diamonds. If cheap diamonds could be made to arbitrary dimensions, then all sorts of "Diamond Age" technologies could make things more durable or significantly better. (eg. Diamonds could replace silicon as a semiconducting substrate in electronics, and diamond touchscreens would be pretty scratch resistant.) Here are a few uses of diamonds that could provide a bit more value than gemstones.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post