1. Expect to find natural defects, such as "silk" or cloudy patches, or _angular_ bubbles in all natural stones. 2. If bubbles are present in artificial material they will be _round_ or rounding. 3. Artificial material will always have _curving_ parallel striæ within it. 4. The _surface finish_ of artificial material is seldom or never equal to that of natural material. It ought not to be necessary to add that material from either source may be cut to any shape, and that artificial rubies may be seen in most Oriental garb, hence all specimens should have applied to them the above tests regardless of the seeming antiquity of their cut or of their alleged pedigree. LESSON XVI HOW TO TEST AN "UNKNOWN" GEM Having now considered separately the principal physical properties by means of which one can identify a precious stone, let us attempt to give as good an idea as the printed page can convey of how one should go about determining to what species a gem belongs. SIGNS OF WEAR IN AN EMERALD. To make the matter more concrete, and therefore more interesting, let us consider a real case, the most recent problem, in fact, that the author has had to solve. A lady of some wealth had purchased, for a large sum, a green stone which purported to be an emerald. After a few years of wear as a ring stone she noticed one day that the stone had dulled around the edges of its table, and thinking that that ought not to be the case with a real emerald, she appealed to a dealer in diamonds to know if her stone was a real emerald. The diamond merchant told her frankly that, while he was competent in all matters pertaining to diamonds, he could not be sure of himself regarding colored stones, and advised the lady to see the author. The matter being thus introduced, the lady was at once informed that even a real emerald might show signs of wear after a few years of the hard use that comes to a ring stone. While emerald has, as we saw in the lesson on hardness, a degree of hardness rated as nearly 8 (7-1/2 in the table), it is nevertheless a rather brittle material and the long series of tiny blows that a ring stone is bound to meet with will cause minute yielding along the exposed edges and corners of the top facets. This being announced, the first step in the examination of the stone was to clean it and to give it a careful examination with a ten-power lens. (An aplanatic triplet will be found best for this purpose.) COLOR. The color was, of course, the most obvious property, but, as has already been said, color is not to be relied upon in all cases. In this case the color was a good emerald green but a bit bluer than the finest grass green. A very fine Maine tourmaline might approach this stone in color, so it became necessary to consider this possibility. A glass imitation, too, might have a color equal or superior to this.