Probably, the finer permanent tints are due to _gold_, in infinitesimal proportions. Professor Faraday has said that the ruby-tinted glass called “Bohemian” derives its colour from gold in a pure form, finely attenuated, and not from any chemical combination of that substance. A white diamond has far more lustre than one which is yellow or violet-tinted. But in all the sapphires, the deeper the hue, the more brilliant and valuable the stone. Hence the technical term for the colour of a diamond is its _water_. In all these gems the hues (whatever their origin) are homogeneously united with the crystal; and the _modus operandi_ of Nature is a profound secret. * * * * * When we descend in the mineralogical scale, and come to such stones as agates and jaspers, the process which has been followed seems quite capable of being traced—for its antiquity is not so great. The colours which we now find in pebbles more or less opaque, though occasionally lively, exist under very different conditions from those by which they reside in the crystalline gems. It is necessary, however, to consider first, the origin and nature of _pebbles_ as distinguished from casual fragments of stone. This was long held to be one of Nature’s riddles; but as soon as it was experimentally dealt with, it met with a solution in some of its most difficult points. It was early decided that pebbles are _distinct formations_, complete in themselves, except in so far as they have been worn away by gradual attrition. The first difficulty was, how to account for the great hardness of many of our seaside specimens. Although for the most part inclosing an “organism,” which must have been that of some zoophyte, they show no traces, in their present compact texture, of the soft and yielding consistency they must at that time have possessed. The matrix in which they lie, and from which they drop as the ripe nuts fall from a hazel-bush, is seldom or never of so hard a substance as are the pebbles themselves; in many cases the difference is as great as between the teeth and gums of a living mammal. But the evidence of the inclosed organism is conclusive as to a past history; and in all mixed pebbles—and ninety-nine out of a hundred of ours are mixed—it is quite certain that an _impregnation_, or an actual _infiltration_, has taken place.