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BOOK I
7
even this one is not always so. Of all the marbles only some parts of a few pieces of phengites marble are transparent and among the stones, properly named, only selenite, magnetis formed in thin crusts, and gyp­sum, the last two minerals only very rarely. Four congealed juices are transparent, halite, nitrum, alum, and atramentum sutorium. Many gems are transparent and amber commonly so. Some of the transparent gems change color when inclined, for example, eristalis (opal), which changes from white to red. Others may become clearer when inclined as does the Carthaginian smaragdus. Gems which can be scratched with steel, espe­cially those which are spherical and protrude like the eye appear to be white when held in the sunlight and have the same color as the rest of the mineral, although somewhat lighter, when held in the shadow. But of these some are brilliant, others dull; some are lighter, others dark. These differences can be observed in the carbunculus.
Luster, brilliant luster, is found in all genera of minerals and I shall mention only a few. Among the earths creta argentaria has a brilliant luster and the same is true of all transparent congealed juices, all transparent stones, all gems, and the native metals. Luster occurs throughout the entire body of the most valuable gems, magnetis, and the mineral we call armatura. On the other hand some minerals do not have a luster through­out all the body. Certain micas have a luster only within the body of the mineral and similarly misy has a golden luster, creta argentaria, silver. Also there are stones to which they attach very thin foils of pure gold, silver, copper, and other metals in order to increase their luster. Among the lustrous minerals some reflect an image, for example, smaragdus, the carbunculus found at Orchomenus, Arcadia, cepites, cepionides, and hephaestites. All hard stones will reflect an image when polished. Obsidi-anus lapis that is called jet reflects an image which resembles a shadow. Although all minerals have this property, the luster we usually see is more often due to art than to nature. The luster of marbles, gems, and metals is the result of polishing. A luster can be given to glutinous earths by mere­ly rubbing them with the finger nail.
I now take up taste. Some minerals have a sweet taste, for example, melitites and galactites; Samia earth and marl, an oily taste; nitrum, a bitter taste; halite, a salty taste; lime and spodos which is found in mines, an acrid taste; red ocher, an astringent taste; and certain earths which have absorbed an acidulous juice, an acidulous taste. Some minerals have a mixed or confused taste, for example, atramentum sutorium and related species which have both an astringent and acrid taste. One perceives the taste of congealed juices by placing them on the tongue, especially salt, alum, soda, iron sulphates, and related species. Earths which have ab­sorbed these juices, as well as stones, are tested in the same way. Astrin­gent earths adhere to the tongue. Some earths, if not all, when placed in a vessel, covered with sweet water, and then worked with the hands, give their taste to the water. The water will be found to have the same taste