Your setting is magical. Clerics are servants of gods. With that in mind:
Anywhere you please
The presence or absence of diamonds in your world isn’t limited by such mundane factors as where volcanoes exist or whether ancient carbon deposits lie buried beneath the surface. You can literally have the Goddess of Diamonds decide to plop a seam of diamonds anywhere. Perhaps a wizard accidentally blew himself up and scattered diamonds across the landscape, or Earth Elementals decided to prank the surface dwellers by putting huge diamond deposits under a loamy flood plain.
Point is: literally anywhere can be a suitable location for your diamond mines as you have so many more justifications available for why the diamonds are buried there. Use your imagination, remember why you said they were there, then build that into your world in an inversion of the usual Worldbuilding ‘my things are here because my world says so’ to ‘my things are here, so my world must be this’.
Goddess of diamonds? Drop a buried temple into the mine.
Ancient Accident? The diamonds are cursed and the wizard’s ghost is still around.
Earth Elemental Jokers? They keep playing tricks on the miners.
Or you can go the route of boring old plate tectonics if you really wish...
If you do, the answer is still ‘pretty much anywhere’, as diamonds can form and be transported to the surface anywhere there has ever been an upwelling of material from the mantle (specifically a rapid form of eruption that transports the freshly made diamond up before they melt again). These kinds of upwelling can happen damn near anywhere that the crust was once thick enough to support it, but then the diamonds can hang around while the geology around them is modified by... well, geology.
Tiny amounts of diamonds can also be made anywhere a load of carbon rich material got trapped in a subduction zone, or even where meteors have impacted. In those cases the diamonds will be very small, but if your world only cares about the worth of the diamond (who decides if the diamond is worth 10GP??) and uses some form of free-market economy then even small diamonds will rapidly become worth mining.
You get the point. Put the diamonds where it works for your campaign. Justify their presence there later.