On Minerals

of obscure transparency to a greater or lesser degree, whose color is not caused
except from the transparent substance, or something composed from it, or mixed with obscure earthy matter
in part or in whole. Because it is easy to know the cause of the color of these stones from what has been said,
Cause of blackness in stones I shall pass over them. Black color in stones, however, is most frequently
caused by burnt earthy matter; for which reason black stones are also frequently
very hard, and are more capable of being polished than they are of being cut. For this
color is not caused except by the privation The absence or lack of a quality of transparency in the mixture, as will be clear
when the science of colors is discussed. Medium colors, however, are red in
its general type, green, and yellow, and their different varieties. And as is said in the book On the
Cause of redness Senses, red occurs when a thin, ignited smoke original: "fumus... succensus"; medieval physics often explained colors as types of "smoke" or vapors trapped in a medium is infused
upon a luminous transparent substance. This is found in certain stones
which they call "watery hyacinths" likely a variety of zircon or pale sapphire, and in three kinds of
Aristotle carbuncles; for which reason, according to Aristotle, all of those are said to be naturally hot.
They differ, however, in their redness; for if there is great transparency and the
infused smoke is very thin and bright, its color is that which is called
balas-ruby original: "palacius". But if there is great transparency and the smoke is like
a thick, ignited fire, then its color is that which is truly called a
carbuncle; and therefore that stone which truly achieves its species shines in the darkness like
a glow-worm original: "noctiluca", and especially when clear and limpid water is poured over it. If,
however, the transparency is somewhat thick and the smoke floating upon it
is somewhat darker, its color will be what is called a garnet,
because it is colored in the manner of a pomegranate seed original: "grani maligranati"; and all three of these
Which are carbuncles according to Aristotle Aristotle calls carbuncles. He says that the garnet is the noblest and driest among these,
even though it is of lesser price among lapidaries original: "exclusores" and craftsmen.
That stone which by some is called a "watery hyacinth" has a color
which is composed of a limpid, watery transparency—not an airy one—and has an
infused, ignited watery smoke, such as exists in a cloud in the sky, and not in the
dawn. In the same way, we find the colors of transparent stones
which are of a yellow color. For if a stone is of a transparent substance and very
clear, and has mixed into it a very fine, strongly burnt earthy matter, then it will be the
color of the clearest sapphire; and it will have different colors according to whether that
transparency with that fine burnt earthy matter is clearer or darker.
For a clear and pure yellow color is undoubtedly caused by great
transparency, because the sight penetrates into it so that the light does not obstruct the vision,
nor does any luminous thing. If, however, with such burnt earthy vapor there is
a watery transparency that is somewhat thick, then the color of the hyacinth results, which
is significantly less clear than the color of a noble sapphire. A sparkling and
sky-blue color original: "ceruleus" is caused by a bright transparency that is covered by a thin and slight
ignited watery vapor; and this is the green color that is called topaz In the Middle Ages, "topazion" often referred to a green stone, possibly peridot. And in
whatever stone veins of golden splendor are found, such as in chrysoprase and
chrysolite, those veins are generated from the same cause. There are, however, many

transparent stones that are green, such as the emerald original: "smaragdus" and chrysolite, and
the stone called "prase" original: "prama", although there is diverse greenness in them.
Cause of green color The colors of all of these are generated from one and the same cause: for it is from a watery
transparency with strongly burnt earthy matter; and according to whether this is clearer or
less clear, that greenness also becomes clearer or less clear.
A proof of this is found in glass that is made from a mixture of lead; for this is very
green, and it is made purer the more often and the more intensely it is burned.
For in repeated burning, it purifies the transparency and refines it, and the
clarity of the fiery light is more infused into its watery nature, and therefore it clarifies it.
The medium color which is nearly tawny and blue-gray, such as is in the stone
called carnelian original: "corneola", is caused by a finished transparency that is infused over
a thick, smoky, watery substance, and similarly over burnt earthy matter. And these are nearly
all the colors that are found in precious stones. After this is the color
of onyx and the sparkling snowy color by which the stone called the
"Orphan" original: "orphanus," a legendary gem in the crown of the Holy Roman Empire is said to be colored. For the onyx stone is composed of a substance that
is of a dual color; and it is sometimes found with more colors than
two, but frequently it is composed of a dual color, one of which
floats over the other. The lower one indeed is like flesh, which is from earthy
matter very smokily mixed as a vapor. The upper one, however, is somewhat pale
and dusky; and this is caused by the victory of transparency over opacity in
the mixed parts, so that the opacity is what alters the white. And such a
substance occurs when a watery thing mixed with a little fine earthy matter evaporates
and, in evaporating, coagulates into a stone. Onyxes are also found
having very red paths and very white ones, the causes of whose color it is not difficult
to find from the things said before. A sparkling snowy color undoubtedly is caused
from the fact that it is mixed from a transparent body that is, as it were, fixed. For
every transparent powder is found to be very white; and when the powder is continuous,
a gleaming white body results, such as a pearl original: "margarita", which happens from the
beating of light upon the surfaces of the polished parts. For this reason, the stone is said
to shine somewhat in the darkness like a glow-worm. For in the day, the light
that is incorporated into the transparent part of the stone is covered because of the greater light; but at
night it shines; and therefore in the day that stone is seen as white as a glow-worm.
A perfect explanation for all of these must be given in the science of the
generation of sensible things. A stone is also found of very many colors, for which reason it is also
called the "Panther" stone, all of whose colors are caused by the diverse substances from
which its parts are composed: for the reason is the same in the whole and in the part
with regard to the tinting of the body. Thus, so many and such things have been said regarding the science of colors
found in the aforementioned stones. The amethyst original: "amctistus", however, follows the
ruby in its dark transparency. And chalcedony follows beryl in its
substance, which is, as it were, a yellowish and dreggy transparency, just as lead imitates
silver.