176 ROCK CRYSTAL. parency. From its hardness and toughness it forms an excellent material for the engraver, and is worked into vases, brooches, and ornaments for the person. Fine specimens are procured from Monte Verdi in Tuscany, the amygdaloidal volcanic rocks of Iceland, and the Faroe Islands. Specimens of sponges in chalcedony from the Chalk formation are found on the sea-coast at Bognor, Selsey, Littlehampton, and Worthing. 1 (d) Agate. (From ay aw, to admire), or from the B. Achates, whence, according to Theophrastus, agates were first brought. 2 Translucent coloured owing to combinations of common quartz, amethyst, jasper, carnelian, and other varieties of quartz, arranged in alternate stripes, or irregularly mixed together. 3 The moss agate is a well-known variety, with moss like delineations due to oxide of manganese or iron. The chief sources are India and the Brazils. (e) Jasper, derived from the Greek ’'lao-iris, is another of the many forms under which quartz is presented to us. It differs from agate chiefly in being opaque. Its colours are extremely varied, and are due to the presence of foreign substances, chiefly oxide of iron; which is, indeed, the great colouring agent which nature employs for stones as well as 1 Bristow, Glos. Min. p. 75. 2 Ibid. p. 4. 3 B. von Cotta, Rocks class, p. 6.