On Minerals

Chapter three of the second treatise: On the colors of stones that are transparent but not "terminated" In this context, "terminated" refers to stones that lack a distinct crystal habit or geometric shape and are not precious.

Lapidary science tells us that those stones found to be transparent yet not terminated in shape have substances of many colors. Generally speaking, there are four kinds: flints original: "silices", tufa original: "thophi", freestone original: "quadrum," referring to stones naturally or easily squared for building, and marble. In all these kinds, many colors are found: black, bluish-gray, a somewhat greenish hue, and white. Outside the genus of marble, a large red stone is rarely found, though small red ones occur, especially among the varieties of marble. The reasoning for all these colors is the same as what was said in the preceding sections. However, it happens in the varieties of marble that the parts broken away from them glitter somewhat, as if they were mixed with metals. This occurs because a transparent substance enters into their composition; when the surface of this substance is thickened, it shines or glitters in those parts. This is one of the reasons why the genus of marble is considered more noble than other kinds of stones. Black, however, is frequently caused in such stones by a "smoky" earthy mixture original: "fuliginoso," referring to the Aristotelian "smoky exhalation" that has been coagulated. White arises from a very fine earthy matter mixed with much water; for this becomes white when "cooked," just as the earth of chalk original: "creta" does in lime and milk. Bluish-gray is caused by an opaque earthy matter that has slightly altered the white that began to coagulate from fine earth and much water. Green, in every kind of stone, is caused by much watery moisture and vapors bedewing the mixture together as it coagulates. Quartz original: "Quarz", however, is a kind that possesses several or all of these mixtures in its parts because of many types of materials gathered together in that place. Tufa stones are frequently either the color of earth or white, like pumice. This kind of stone is generated from earth in which water foams while mixed with that same earth; therefore, when it is coagulated by a heat of digestion called optesis original: "optesis," a Greek term for "roasting" or "baking" in Aristotelian physics, it is found to be spongy and light. Pumice is made from much water whose foam has suffered intensely from the earthy matter mixed with it; it is white because of the whiteness of such foam. Among the kinds of marble, the white variety called alabaster is composed, without any doubt, of much transparent matter that has been altered and intensely acted upon by fine earthy matter; thus, a most noble, shimmering color results in it. That which is called porphyry marble has the color of dark flesh with white spots; we have already explained the cause of such a color. Flints, however, are for the most part a brownish-dark color, the cause of which has also been sufficiently assigned. Let what has now been said suffice, therefore, concerning the colors of stones.

Chapter four: On the cause of the various degrees of hardness found in stones.

We shall speak consequently of the diversity of hardness found in many varieties of stones. For all kinds of precious stones are so hard that a file removes nothing from them, and when struck hard against one another, as against hardened steel, they emit fire. On the contrary, almost every kind of tufa is found to have so little hardness that it can be carved with a small tool. Furthermore, certain white stones commonly called chalk, and some even softer and whiter, are found to have the least hardness among the genus of stones. Every kind of flint is extremely hard, and after these, marble in its own kind. Freestone is medium in the hardness of stones; nevertheless, greater and lesser hardness is found within it. This commonly happens to the harder stones: if they lie in the air for a long time during cold weather, they afterwards split into many parts when exposed to the sun. On the other hand, those that are less hard—unless they are poorly mixed, like burnt lime—ought to become better and harder the longer they lie in the air in buildings, provided the coagulating power is present; and they do not split from the cold. It is the task of physics to assign the cause of these accidents from the material and the efficient cause, just as was said in other cases. We say, therefore, that the general cause of hardness is dryness.

Cause of hardness

Since "hard" is that which has a natural power to resist a touching object, and "soft" is that which by nature lacks such power, hardness cannot be caused except by the "dry" quality, which stands firm in itself without yielding to another. In the nature of stones, dryness is caused by two things, as was shown above. Either it is because heat has drunk up the moisture from earthy matter, and thus the earth remains hardened; or it is because extreme cold has seized the transparent moisture and, by turning it toward its own property, has squeezed out the moisture. By violently compressing the matter, the cold has both hardened and excellently bound it together, as is seen in transparent stones. Because of this, they are extremely hard; when struck, they emit fire and do not endure the file, but must instead be polished by grinding and rubbing with emery original: "amsi," likely a corruption or variant for abrasive powder/emery.

Another cause of hardness.

In other stones made of earthy matter, the cause of greater hardness is nothing other than greater aridity. This is caused by a stronger or less strong heat on the part of the efficient cause, and by moisture that is more or less easily separable from the material. For very oily moisture is easily held together, while entirely watery moisture evaporates easily. Because of this, stones that are like chalk, or softer than chalk, are very white and leave a white stain on things they touch; they are certainly mixed with a very evaporable moisture and have been "burnt" by an exceeding heat beyond the measure of coagulation. They have already begun to turn into lime original: "calinari," i.e., calcination, which is why they do not remain in walls; being externally rough like dried-out lime, this roughness recedes into the tenacity of the cement and leaves another part of the stone glued to the cement. Therefore, these stones fall from the walls, and the wall falls from them after a short time while the material is drying out.

Why flints are the hardest

Flints, however, are the hardest because their moisture, which is not separable from the matter, has evaporated greatly and has been hardened by a violent earthy aridity. And therefore they also... The text cuts off mid-sentence at the end of the page