PURPOSE: To form a colorful thin noble metal film on the surface of diamond by dissolving an org. compd. of a noble metal and an org. compd. of other specified metal in an org. solvent or further dissolving a resin binder, coating the surface of the diamond with the resulting soln. and carrying out baking.

CONSTITUTION: An org. compd. of a noble metal such as Pt, Au or Ag synthesized by allowing the chloride or other inorg. salt of the noble metal to react with org. carboxylic acid and an org. compd. of a metal such as Cr, Sb, Pb, Bi, Si, V, Ba, Zr, Ca or Al synthesized in a similar way and used to obtain various color tones as well as to enhance the adhesive strength of the noble metal to diamond are dissolved in an org. solvent such as methanol or toluene or a binder such as epoxy resin is further dissolved. The cut faces of diamond are coated with the resulting soln. and this soln. is dried and heated to ≤600°C to decompose the org. compds. A colorful thin noble metal film is formed on the cut faces of the diamond with superior adhesive strength.