Moonstone, ruby and diamond ring, 1920s. Moonstone is a variety of the mineral feldspar, and is famed for an optical phenomenon known as ‘adularescence’ – a ghostly flash of blue, white or occasionally pale yellow that moves across the stone when viewed from certain angles. This cabochon moonstone displays a dreamy pale blue sheen, and sits at the centre of an unusual 1920s rectangular mount, millegrain-set with beautiful antique rose-cut diamonds diagonally crossed with calibre-cut rubies, which have been specially cut to fit the mount.