Are diamonds in rocks

Yes, they are found in rock called kimberlites. They are also found in some meteorites.

Another Answer

Kimberlite is a trace mineral that may indicate diamonds are also present in the volcanic pipe's erupting debris. Some raw stones are found loose, others are attached to other erupted material.

Often, in the sorting process that takes place at the mine, raw diamonds are separated from other material by tumbling and washing with water.