Diamond is used everywhere in tons of applications. It's especially useful in cutting and drilling of metal and stone. Although contra to the GP's point, industrial diamonds are in a completely separate market from gemstone diamonds and de beers' monopoly doesn't have much effect on that. You can find diamond tipped and coated tools at any local hardware store, for example, at quite reasonable prices.

Diamonds are also useful for cutting stones. Granted this was never much of a problem because one only needs to sprinkle the saw blade with tiny diamond fragments. Your local hardware store should have a shelf of diamond cutting blades.

In addition to hardness, they have an exceptionally high thermal conductivity.

There's also some quantum computing applications, but that requires synthetic diamonds doped with some other elements, and in very thin sheets. The energy levels of the electrons around the impurities are the different states it can be in, and can be controlled by lasers.

Apart from the thermal properties mentioned already, and other more esoteric uses, they also happen to be really strong, and if we can scale up our manufacturing processes, they should become quite cheap as well.

Imagine all of the applications for a transparent material stronger than steel! We're a way off yet, but it's feasible.

Sharpening. Almost anything that requires abrasion benefits awesomely when the abrasive is diamond.

Imagine if you could cover or just replace every fragile piece of anything with diamonds.