The interesting thing about a 'diamond' planet is that it would be a waste of time. Diamond's value is superficial - it can't actually be used for anything (except cutting), so mining this planet would essentially cause diamonds to become worthless.

Whats more interesting is a planet made up of rare metals - that would still cause the prices to drop (this assumes the mining can easily generate lots of metal easily, transport costs aside), but can immediately be turned around into something useful.