CHAP. 26.—DEFECTS IN CARBUNCULUS, AND THE MODE OF TESTING IT.
Nothing is more difficult than to distinguish the several varieties of this stone, so great an opportunity do they afford to artistic skill of compelling them to reflect the colours of substances placed beneath. It is possible, they say, to heighten the brilliancy of dull stones, by steeping them for fourteen days in vinegar, this adventitious lustre being retained by them as many months. They are counterfeited, too, with great exactness in glass; but the difference may be detected with the touchstone; the same being the case also with other artificial stones, as the material is always of a softer nature and comparatively brittle. When thus tested by the stone, hard knots, too, are detected in them; and the weight of the glass counterfeit is always less. In some cases, too, they present small blisters within, which shine like silver.
CHAP. 27.—AHTHRACITIS.[3093]
There is also a fossil stone found in Thesprotia, known as “anthracitis,”[3094] and resembling a burning coal[3095] in appearance. Those who have stated that it is a native also of Liguria, are mistaken, in my opinion, unless perhaps it was to be found there in their time. Some of these stones, they say, are surrounded with a vein of white. Like those which we have mentioned above, they have a fiery colour, but there is this peculiarity in them, that when thrown into the fire they have all the appearance of becoming quenched and deadened; while, on the other hand, if they are drenched with water, they become doubly glowing.[3096]