''Wonderstone'' ENDOR cavity.
We report the design and construction of a high-quality ENDOR cavity from a naturally occurring rock, ''Wonderstone.'' It consists of a cylindrical cavity operated in the TE(011) mode, with four vertical posts forming the ENDOR coil. 100 kHz field modulation is achieved by external coils mounted on the pole faces of the magnet. This cavity is especially suited for the insertion of a cold-finger Dewar. The merits of Wonderstone as a substrate material for microwave cavities and the details of obtaining a fine conducting layer of silver on the inner surface of the cavity are discussed.