It is also curious to note that in some cases the stones have not been caused by aqueous deposit in an already existing hollow, but the aqueous infusion has acted on a portion of the rock on which it rested, absorbing the rock, and, as it were, replacing it by its own substance. This is evidenced in cases where gems have been found encrusted on their matrix, which latter was being slowly transformed to the character of the jewel encrusted, or "scabbed" on it. The character of the matrix is also in a great measure the cause of the variety of the stone, for it is obvious that the same salt-charged aqueous solution which undergoes change in and on ironstone would result in an entirely different product from that resting on or embedded in silica. Following out the explanation of the aqueous solution, in which the earth-crust constituents are secreted, we find that the rarer and more precious metals do not generally enter into the composition of precious stones--which fact may advisedly be repeated. It is, of course, to be expected that beryllium will be found in the emerald, since it is under the species beryl, and zirconium in zircon; but such instances are the exception, and we may well wonder at the actions of the infinite powers of nature, when we reflect that the rarest, costliest and most beautiful of all precious stones are the simplest in their constituents. Thus we find the diamond standing unique amongst all gems in being composed of one element only--carbon--being pure crystallised carbon; a different form from graphite, it is true, but, nevertheless, pure carbon and nothing else. Therefore, from its chemical, as well as from its commercial aspect, the diamond stands alone as the most important of gems. The next in simplicity, whilst being the most costly of all, is the ruby, and with this may be classed the blue sapphire, seeing that their chemical constituents are exactly the same, the difference being one of colour only. These have two elements, oxygen and aluminium, which important constituents appear also in other stones, but this example is sufficient to prove their simplicity of origin. Another unique stone is the turquoise, in that it is the only rare gem essentially containing a great proportion of water, which renders it easily liable to destruction, as we shall see later. It is a combination of alumina, water, and phosphoric acid, and is also unique in being the only known valuable stone containing a phosphate.