Here's a couple of methods that may help. If either method appeals to you let us know and we can give you more in depth direction on what can potentially be complicated topics.
Do you have an image or a pattern of diamonds that you like? If so you could add that as a layer to your document and then apply a mask to that layer. Using black and white you could then paint on that mask to reveal or conceal different parts.
If you are wanting to paint on diamonds in a more spread out method you could use a brush. Find an image of a single diamond and turn that into a brush. Using the Brush Settings panel you can alter attributes like Spacing and Scattering to introduce randomness into the brush. You can then paint diamonds wherever you want them.