JASPER, in natural history, a genus of scrupi, of a complex irregular structure, of a great variety of colours, and emulating the appearance of the finer marbles, or semipellucid gems.
The great characteristic of jaspers is, that they all readily strike fire with steel, and make not the least effervescence with aqua fortis.
Jaspers, though commonly reckoned among the precious stones, ought undoubtedly to be ranged among the scrupi; being only opaque crystalline masses, variously debased with an earthy admixture: and to this last ingredient it is that they owe all their variety of colours, as white, green, red, brown, and bluish.
The several kinds of nephritic stone, and the lapis divinus or jade, are all genuine jaspers; but the hard, bright, green jasper of the East Indies, seems to be the true kind. It is found in masses of various sizes and shapes: but the more usual standard as to size is between four and six inches in diameter; tho' there are masses of it found of a foot or more in diameter, and others no larger than a horse-bean. It is generally simple and unmixed: but if it be variegated at all, it is always with white; and this is disposed not in streaks or veins, but in clouds. It is capable of a very fine polish; and, when the white clouds are well disposed, is very beautiful; and, in pieces not too thick, is tolerably pellucid when held up against the light.