Agates, therefore, grow from the outside inward. The process may go on until the cavity is entirely filled or may cease at any time. If water remains in the cavity for some time crystals, such as are sometimes seen, will be formed. The nodule of silica or agate formed by the percolating waters is harder and more resistant than the surrounding rock. Hence it remains after the surrounding rock has been worn away. We can thus understand why agates should be found, as they usually are, on sea or lake beaches, or in the beds of streams.

The different colors seen in the natural agates are produced by traces of organic matter or of oxides of iron, manganese or titanium contained in the waters which formed them.

The beautiful moss-like inclusions seen in the moss agates are due to a partial crystallization of oxide of manganese or iron contained in the waters. The particles of oxide in these cases arrange themselves in arborescent forms, just as do the particles of frost crystallizing on a window pane.

Agates are not used as extensively as they once were for ornamental purposes. In the years of 1848-50 agate jewelry was very fashionable and was extensively worn. At the present time, however, the principal use of agate in jewelry is for breast pins and watch charms. For ornamental purposes it is used in pen-holders, knife handles, and vases. Its use for large marbles was once quite common, but glass marbles of the same size and still called "agates" are now generally substituted. In fine mechanical work, such as bearings for delicate instruments and in tools for polishing and grinding, agate is still extensively used.