[awm]

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.

[scarmig]

Rare earth metals are, despite the name, not actually all that rare. They're fairly abundant in the Earth's crust and mantle.

The reason they're more expensive than, say, iron, isn't that they're rare but they're rather difficult to purify from each other. They share the same outer shell electrons (s and d), and what separates them from each other is their f-shell electrons. Those, however, are buried inside the other shells. So their differences are largely driven by differences in atomic radius, differences which are small in chemical effect.

Which is all to say that the cost of producing them wouldn't change too much in that scneario, even if you had access to a trillion tons of ore for free.

[taejo]

And the diamonds we do need for cutting can be synthesised on Earth pretty cheaply.

[TNO]

It has uses beyond cutting: <http://www.pbs.org/wgbh/nova/tech/butler-diamonds.html>;