Watching my kids play Minecraft, I was reminded of a children's novel I enjoyed many years ago called Twenty-One Balloons about a clan that discovered a vast deposit of diamond on a small volcanic island.
Suppose that natural diamond was found on earth in such concentration and abundance as coal. Could that enable any technology or have any discernable impact on modern society? (Other than to remove diamond from use as a store of value?)
I'm having a hard time imagining any significant benefits that might accrue from convenient access to large diamond crystals. Although diamond is extremely hard, it is also quite brittle, so I don't think that any large cutting tool could be fashioned from solid diamond that isn't already made (and more durably) from industrial diamond specks bound to metal substrates.
I could imagine diamond being a luxury building material, like other polished stone slabs and blocks. Indeed, the fundamental difficulty of cutting and working it would probably keep its status as the most luxurious architectural material, even if crushed diamond was otherwise being used as a fuel like coal.