Silica
oxide of silicon. Pure silica occurs in nature, crystallized in six-sided in rock crystal and some other forms of prisms, terminated by six-sided pyramids quartz. It enters largely into the composition of agate, chalcedony, flint, opal, other minerals. In a perfectly pure state it is quite transparent and colorless. Its other minerals. In a perfectly pure state hardness is next to that of the precious stones, and it has a sp. gr. of 2.6. Silica presents the general characters of an earthy base, but acts as an acid, forming with the bases compounds known as silicates. These are very abundant in naa large number of other common minerals ture; clay, felspar, mica, hornblende, and are compounds of this description.