# A Treatise on Diamonds, and Precious Stones/Chapter 1/Section 4

SECT. IV.
Physical and Chemical Characters of the Diamond.
It is the object of the present chapter to point out to the reader such of the peculiar characteristics of the Diamond as may enable him to distinguish it from any other substance, especially in its rough state, and consequently to detect any imposition which may be attempted to be practised upon him.
Rough Diamonds occur under the most beautiful and regular geometric forms, their lustre and color frequently resemble Gum Arabic; they often exhibit a polished appearance, more particularly the brown stones, others are dull, indented, rough, shewing their Jamine, and some are round. If, as might ​possibly be the case, a stone of the latter description was shewn toa person who had never seen a Diamond, although he might not be able to decide what it was, yet after examination he would pronounce it to be very unlike, and very different from any pebble he had met with before.
The most common form of rough Diamonds is the octahedron, variously modified on the edges with planes, and low pyramids on each face of the triangles. The next is the dodecahedron, which occurs also variously modified, both on its edges and planes: others are flat and triangular (heliotrope),these are termed veiny, and being thin, are generally split and cut into roses. The rarest form is the cube. No substance in the Mineral Kingdom displays in miniature such beautiful variety of regular solids as the Diamond, which are as captivating to the connoisseur, as the finest brilliant can be to the wearer.
​Generally speaking, the degree of polish which a stone is capable of receiving, depends upon its hardness. That of the Diamond, which renders itso difficult to be cut and manufactured, enables it to retain unimpaired the lustre which it has originally received.
Small particles, laid on a new anvil and struck with a hammer, will indent both, and in some. cases will not be broken. Yet with this intensity of hardness the Diamond is far from being: difficult of fracture: a slight blow often causes it to split, and the fine edge of a brilliant is frequently injured by clumsy workmen. I have, known one to be chipped by falling on a boarded floor.
A regular formed Diamond, iflightly struck in the direction of the laminæ, will split easily; indeed it admits of cleavage in fourdirections, and I possess a perfect octahedron formed by cleavage. Spheroidal Diamonds break with ​a rough fracture; these are generally called Diamonds of nature. The fracture of this gem is the criterion sometimes relied on by the overseers of the mines in Brazil. Whena stone is delivered to them of a rude appearance, both ill formed and of a bad color, they have recourse to the hammer, especially if it exceed in size the Diamonds usually met with; they hold it upon a hard substance, and give it a sharp blow, should the fracture prove distinctly lamellar they are convinced that it is a true Diamond. In the treasury at Rio de Janeiro, I noticed three or four flat pieces of a bad brown color, an inch long by half an inch in breadth, and about one eighth in thickness.—To this rough kind of experiment may be attributed the fragments occasionally met with.
Diamonds may be distinguished by the senses of hearing and feeling, in the following manner. If two diamonds be held between the fingerand thumb, and rubbed strongly together, a peculiar grating noise will be pro​duced accompanied with a particular sensation on the nerves in contact with them—Or, if a diamond and a pebble, of equal size, be rubbed together in the same manner, the diamond will not be easily moved, as its projecting surface will penetrate the pebble.
To ascertain the supposed phosphorescence of diamonds | have tried many experiments (described by authors) without any satisfactory result, nor have my friends had any better success, even in a single instance.
The combustion of the Diamond is a property very generally known. It burns with great brilliancy underaflame urged by oxygen gas; and if exposed on a support of clay to a strong heat, it may be totally consumed.
​The following experiment is worthy of being recorded. A small portion of iron was put into a crucible with a fragment of a Diamond, and exposed to a strong heat; on examination the iron was found to be converted into steel.
Diamonds of bad color are often improved by being exposed to a certain degree of heat in a small crucible, or the bowl of a tobacco-pipe, surrounded in some instances with borax. It is necessary for the vessel to be closely filled with charcoal, to prevent the admission of air.
The Diamond possesses double refraction; which is rather difficult to discover., By friction it becomes electric, but continues so only a few minutes.