This paper proposes the use of polishing suspensions based on ultradisperse diamond powders of a detonation nature for superfine finishing of elements for X-ray optics. A technique for determining the roughness with subnanometer resolution is described in detail. The following roughness values are obtained during polishing: 3 AA for fused quartz, 5 AA for K8 glass, and 10 AA for nickel electrochemically deposited on copper.