The pear-shaped brilliant which was first mentioned was of a peculiar, slightly yellowish, red color. It was very pellucid and free from any striæ either of the straight or curved types. It had in fact no flaws except a rather large nick on one of the back surfaces near the girdle. This was not in evidence from the front of the stone and had evidently been left by the Oriental gem cutter to avoid loss in weight while cutting the stone. The peculiar yellowish character of the red color led us to suspect ruby spinel. The stone was therefore inspected with the dichroscope and found to possess no dichroism. The sunlight-card test, too, showed that the stone was singly refracting. A test of the hardness showed that the material barely scratched topaz, but was attacked by sapphire. It was therefore judged to be a red spinel. The large bluish stone which the gentleman acquired with the red stones proved to be iolite, sometimes called cordierite or water-sapphire (_Saphir d'eau_), a stone seldom seen in this country. It had marked dichroism--showing a smoky blue color in one direction and a yellowish white in another. The difference was so marked as to be easily seen without the dichroscope. LESSON XVII SUITABILITY OF STONES FOR VARIOUS TYPES OF JEWELS, AS DETERMINED BY HARDNESS, BRITTLENESS, AND CLEAVABILITY HARD STONES NOT NECESSARILY TOUGH. As was suggested in the lesson on hardness there is prevalent in the public mind an erroneous belief that hardness carries with it ability to resist blows as well as abrasion. Now that _it does not follow that because a precious stone is very hard, it will wear well_, should be made plain. Some rather hard minerals are seldom or never used as gems, in spite of considerable beauty and hardness, because of their great brittleness. Other stones, while fairly hard and reasonably tough in certain directions, have nevertheless so pronounced a cleavage that they do not wear well if cut, and are sometimes very difficult to cut at all. In view of these facts it will be well to consider briefly what stones, among those most in use, are sufficiently tough as well as hard, to give good service in jewels, such as rings, which are subject to rough wear. We may also consider those stones, whose softness, or brittleness, or ready cleavability, requires that they should be reserved for use only in those jewels which, because of their nature, receive less rough usage. In order to deal with the principal gems systematically, let us consider them in the order of their hardness, beginning with the hardest gem material known, which is, of course, diamond.