I'm trying to create something similar to the first image and I'm not sure how to go about creating that transition between the granule portion and the black tar portion as simply as possible.

Approximate goal Approximate goal render preview Where I'm currently at Current reality View of the mesh. Just a cylinder with one material applied to the top set of faces and another applied to the lower ones. granule texture An example of the granule textures I'm working with.

I need to keep the modeling relatively simple since there will be a whole series of these with different color granules and the details in the black after the transition will be slightly different.

Let me know if there's any more info I can provide to make this easier to figure out.