Complete Works, Vol. V

glisten somewhat as if they were mixed with metals. And this happens for this reason: because into their substance enters mixed transparent material, whose surface, when it is thickened in the parts, shines or glistens. And this is one of the causes why it is a nobler genus of marbles than other genera of stones. Black, however, is frequently caused in such a genus of stones by mixed and coagulated sooty earth. White, however, is from very subtle earth that is mixed with much watery material; for this turns white when cooked, just as earth does in cheese and milk. Sea-green, however, is caused by opaque earth that has altered a little white, which began to coagulate from subtle earth and much watery material. Green, however, in every genus is caused by much watery material, mixed with vapors simultaneously dewing it, and thus coagulated. The genus of drops likely referring to agate or stones with orbicular inclusions is one that has several or all of these mixtures in its parts, due to the many kinds of matter gathered in that place. Tufa is frequently either of an earthy color, or white, like pumice stones. For this genus of stone is generated from earth in which water foams and is mixed with the same earth; and therefore, when it is coagulated with heat by the digestion that is called optesis baking, it is found to be spongy and light. Pumice is made from much water, whose foam has suffered vehemently from the earth mixed with it; and therefore it is white due to the whiteness of such foam. In the genera of marbles, the white that is called alabaster is, without any ambiguity, composed of much transparent material that has been altered and has suffered vehemently from subtle earth; and thus a most noble, glistening color results in it; this, however, which is called porphyry marble, has the color of dark flesh with white little dots; and we have already said the cause of such a color. Flints are for the most part of a dusky color; the cause of which has also been sufficiently assigned. Let what has now been said regarding the colors of stones suffice.

CHAPTER IV.

On the cause of the diverse hardness that is in stones.

We shall say subsequently regarding the diversity of hardness that is found in many diversities in stones. For all kinds of precious stones are so hard that nothing is taken from them by a file, and when they are struck forcefully against themselves or against hardened steel, they emit fire. On the contrary, almost every genus of tufa is found to have so little hardness that it is cuttable with a small instrument. However, certain white stones that are commonly called creta chalk, and certain others that are found to be softer and whiter, have less hardness in the generation of stones. Every genus of flint is very hard, and after these are marbles in their genus. Square stone is intermediate in the hardness of stones, and yet in it is found greater and lesser hardness; but this happens commonly to the harder stones, which, lying for a long time in the air in time of cold, are afterward split into many parts by the sun. On the contrary, those that are less hard, unless they are poorly mixed as if in the manner of burnt lime with the baking optesis coagulating them, become better and harder the longer they lie in the air in buildings, and they are not split by cold. It is the task of the physicist to assign the cause of these accidents from matter and the efficient cause, just as was said in other things. Let us say, therefore, that the general cause of hardness is dryness. For since that is hard which has the natural power to resist the touching thing, and the soft has the impotence of such nature, it cannot be caused except from the dry that stands in itself not yielding to another. Dryness, however, is caused from two things in the nature of