Brought Italian rock-crystal cutters from Milan to take control of the crystal and glass-cutting works he had established at Prague.
Air, flint, glass, rock-crystal, calcareous spar were examined, but without effect.
Clear transparent rock-crystal is used for optical purposes and spectacle lenses.
Further, these varieties may be of almost any colour, whereas transparent crystals have only a limited range of colour, being either colourless (rock-crystal), violet (amethyst), brown (smoky quartz) or yellow (citrine).
A few magnificent specimens of rock-crystal twinned according to this law have been found at La Gardette in Isere, and in Japan they are somewhat abundant.