STRUCTURAL DEFECTS OF SCIENTIFIC STONES. So far our tests have been mostly negative. It was said, however, that spherical bubbles sometimes appear in scientific gems. Another characteristic _structural_ defect of practically every scientific gem may be utilized to distinguish them. As is well known, the rough material is formed in boules or pear-shaped drops under an inverted blowpipe. The powdered material is fed in with one of the gases and passes through the flame, melting as it goes, and then accumulating and crystallizing below as a boule. The top or head of this boule is rounding from the start, and hence the successive layers of material gather in thin curved zones. The color and structure of these successive zones are not perfectly uniform, hence when cut stones are made from the boules these _curving_ parallel layers may be seen within by the use of a good lens, especially if the cut stone is held in a strong crossing light, as was suggested when directions were given above as to the best way to look for "silk" in a natural stone. Owing to the shape of a well cut stone it is sometimes difficult to get light through the material, yet by turning the stone repeatedly, some position will be found in which the curving parallel striæ can be seen. They are easily seen in scientific ruby, less easily in dark blue sapphire, but still they can be found on close search. In the light colored stones and in white sapphire, the difficulty is greater, as there are no color variations in the latter case. However, the value of white sapphire is so slight, whether natural or artificial, that it is a matter of but little moment, and what has already been said as to natural defects, applies to white sapphire as well as to the colored varieties, and absolutely clear and perfect natural white sapphire is rare. One more distinguishing mark of the scientific stones may be added to give full measure to the scheme of separation, that no one need be deceived. The surface finish of the scientific stones is rarely as good as that of the natural material and it appears to be more difficult to produce a good polish on scientific stones than on natural ones. The degree of hardness of the scientific stones seems to be slightly variable in different parts of the same piece so that the polishing material removes the surface material unequally, leaving minute streaky marks on the surfaces of the facets. Possibly this condition might be remedied by skillful treatment, but hardly at the price obtainable for the product, so that a close study of the surface finish will sometimes help in distinguishing between natural and artificial material. Any fine specimen of natural ruby or sapphire will have usually received very expert treatment and a splendid surface finish. In conclusion, then, the points to be remembered in determining the origin of corundum gems are four in number.