Whence this mysterious change? the reader may ask. We can only say that the complete transparency of the stone had been lessened, and perhaps the change was due to the action of some of the ingredients of the cement with which we united the fragments of the broken gem. Some of the Central American opals have the reputation of fading and becoming translucent and opaque in course of time, or according to the circumstances of exposure. We will relate an instance which forms a part of our experience and education in the study of gems. A few years ago, two Spaniards arrived in New York with a bag of rough opals brought from Central America, but from what particular locality we never learned. The specimens varied in size from that of a bean to that of an English walnut, and were extremely beautiful. They had a fresh appearance, as though they had been recently extracted from the mines, and many of them had portions of the soft sandy matrix still attached to them. They excited suspicions of not having been properly tempered and hardened by exposure; but their beauty, which reminded one of the perfect glow-worm, or lumps of phosphorus moistened with oil, did not allow the spectator to hesitate about the purchase of them, especially as they were offered at a moderate price. We invested in the purchase of several charming specimens, and never wearied in examining their exquisite effects. Still, we felt a vague suspicion of the enduring qualities of our newly acquired treasures. The most beautiful stone, the size of a small almond, we carried in our pocket for a long time, not only for our gratification but for the purpose of studying the effect of the atmosphere upon its reflections. Soon after our acquisition, we fancied a slight shadow or nebulosity appearing in one end of the stone. We carefully watched it, and before long an indistinct cloudiness began to appear, like the dim and distant haze of a summer sky on the commencement of a storm. Even then we thought it might be mere fancy on our part. But when the shadow changed to opacity, and the transparency of the gem, with its beautiful reflections, vanished, never to return, we were compelled to admit that even substances of the mineral kingdom had their diseases as well as forms of the organic world. This is indeed but one example to illustrate a theory; but most of those we purchased at that time of the Spaniards have altered in appearance, and some of them quite as seriously. Therefore we have arrived at the conclusion that recently mined opals should be bought with caution; and that the perfection of a rough opal as a gem cannot be safely estimated until after it has been cut by the lapidary.