VITREOUS LUSTER. The other stones nearly all have what is called _vitreous_ luster (literally, glass like), yet owing to difference of hardness, and consequent minute differences in fineness of surface finish, the keenness of this vitreous luster varies slightly in different stones, and a trained eye can obtain clues to the identity of certain stones by means of a consideration of the luster. Garnets, for example, being harder than glass, take a keener polish, and a glance at a doublet (of which the hard top is usually garnet and the base of glass) will show that the light is better reflected from the garnet part of the top slope than from the glass part. This use of luster affords the quickest and surest means of detecting a doublet. One can even tell a doublet inside a show window, although the observer be outside on the sidewalk, by moving to a position such that a reflection from the top slope of the stone is to be had. When a doublet has a complete garnet top no such direct comparison can be had, but by viewing first the top luster, and then the back luster, in rapid succession, one can tell whether or not the stone is a doublet. OILY LUSTER. Certain stones, notably the peridot (or chrysolite) and the hessonite (or cinnamon stone), have an oily luster. This is possibly due to reflection of light that has penetrated the surface slightly and then been reflected from disturbed layers beneath the surface. At any rate, the difference in luster may be made use of by those who have trained their eyes to appreciate it. Much practice will be needed before one can expect to tell at a glance when he has a peridot (or chrysolite) by the luster alone, but it will pay to spend some spare time in studying the luster of the various stones. A true, or "precious" topaz, for example, may be compared with a yellow quartz-topaz, and owing to the greater hardness of the true topaz, it will be noted that it has a slightly keener luster than the other stone, although both have vitreous luster. Similarly the corundum gems (ruby and sapphire), being even harder than true topaz, take a splendid surface finish and have a very keen vitreous luster. Turquoise has a dull waxy luster, due to its slight hardness. Malachite, although soft, has, perhaps because of its opacity, a keen and sometimes almost metallic luster. One may note the luster rapidly, without apparatus and without damage to the stone. We thus have a test which, while it is not conclusive except in a very few cases, will supplement and serve to confirm other tests, or perhaps, if used at first, will suggest what other tests to apply. Another optical effect that serves to distinguish some stones depends upon the reflection of light from within the material due to a certain lack of homogeneity in the substance.