Non-contact polishing of a crystalline layer or substrate by ion beam etching

Polishing method comprising the steps of: - providing at least one crystalline layer or substrate, the at least one crystalline layer or substrate extending in at least one plane, and including at least one outer surface and at least one depression extending from the at least one outer surface; and - polishing the at least one outer surface using ion beam etching (IBE) or an accelerated inert gas ion beam, the ion beam being incident on the at least one outer surface at non-normal incidence or at a non-zero angle (θ) with respect to the surface normal of the at least one plane of the crystalline layer or substrate.