Stones to imitate While the raw materials of ceramics (mainly clays and metallic oxides) are indeed children of the Earth's energies, hard stones also offer ceramicists shapes, colours, and textures that the latter manage to imitate in a spectacular way with refractory colours. The green of malachite inspired a ewer at the English manufacture of Minton or cups at the Limoges factory Chastagner. The cloudiness of agate is reproduced in glass or ceramics, while obsidian, a glassy volcanic rock, invites copying its "ribboned" red traces in the glaze. Stones to invent Often, the interest of ceramicists focuses less on the appearance of a mineral than on its capacity to produce, once fired, a sought-after colour. The famous "grand feu" or "kiln" blue is, for example, obtained thanks to cobalt. Copper oxide offers varieties of red, green, or purplish depending on the firing atmospheres. Cassius purple, named after the German chemist Andreas Cassius (1605-1673) who developed it, is a pink obtained from a gold precipitate. The Limoges native François II Alluaud, passionate about mineralogy, invented a black glaze integrating black rocks from the diorite family.