The two types of diamond deposits are referred to as primary and secondary. The primary deposits are forced up to the surface in kimberlite. By the erosion of the easily broken down kimberlite, the diamonds wash down to the rivers and these deposits are called alluvial (or secondary). In many cases, they are eventually carried down-stream — sometimes thousands of miles, to the ocean. These are called marine deposits. Alluvial deposits are usually the more concentrated in diamonds, since the action of the water current separates the heavier minerals from the lighter.