The People of The Shining Isles have long been exploited. They are cruelly used in mines, excavating tonne after tonne of rock and dirt in pursuit of what the Warriors from Away call ‘diamonds’. These small, shining rocks are apparently of great value to the invaders for some reason or another, though the People have always ignored them, preferring to grow vegetables to sacrifice in order to prevent the ancient Smoking Gods from waking.
The question here is pretty simple: Is there a set of circumstances that can lead to high concentrations of diamonds on an otherwise volcanic island chain (think Hawaii), or will the production of such islands always act to destroy the gemstones before they can be mined?
The length of time required is of no consequence (this is an old world), and the number/distribution of tectonic plates is similarly unimportant. The world does have to be earthlike though, so keeping gravity/atmospheric composition the same would be appreciated. I’m open to ridiculously unlikely chains of geologic events leading to this state, but in deference to William of Ockham simpler answers are better.