I feel that I should mention a few points here. One, according to the first result I got when googling "diamond planet":
Quote:
The planet, 55 Cancri e, is twice the size of Earth and so rich in carbon that it holds at least three times our planet’s mass in diamonds, according to a new paper that has been accepted for publication in the journal Astrophysical Journal Letters.
THREE TIMES our planet's mass in diamonds. This is clearly far, far, FAR more than enough diamonds for every man, woman, and child on Earth. Even if we went to this planet and mined enough diamonds for literally every human being alive, it would in no way have any real impact on the planet itself; it would hardly "destroy" it.
Two, the costs involved with building a vehicle that would be able to 1) travel 40 light years, 2) contain the necessary mining equipment and either humans or robots to operate it, and 3) have the cargo space to take enough diamonds back to make it worthwhile, would make this a ridiculous venture and a total waste of money at best. As IronySandwich mentioned, it's relatively simple and cheap to manufacture gem-grade diamonds with materials readily available.
It's certainly an interesting discovery, and could provide new insights into planetary formation and evolution mechanics. I think it would be fascinating to someday visit the planet "in person" to study it in better detail. But mining it? By the time we got there, mining it would be totally irrelevant. I don't think this planet is in any danger of destruction by us.
Oh, and there almost definitely would not be anything living there - apparently the surface temperature is around 2100 degrees C.