At least, we might gather from this passage that the poet was aware of the distinction between ruby and carbuncle (pyrope garnet).
If magnesium takes the place of calcium, then pyrope is formed.
Specimens of pyrope with attached or embedded diamond had previously been found in the blue ground of the De Beers mines.
It seems related to both almandine and pyrope, and shows the absorption-spectrum of almandine.
Two varieties of garnet, almandite and pyrope, may exhibit the dark blood-red color especially ascribed to garnet.
The somewhat less dense, and less hard blood red garnets are properly called "pyrope garnets" (literally "fire" garnets).
The home of the pyrope, however, is and has been for many centuries, Bohemia.
The so-called "Cape rubies" of the diamond mines of South Africa are pyrope garnets and some large and fine ones are found.
Almandite garnet, the "almandine" of the jeweler is less abundant than pyrope, when of gem quality.
In another eclogite boulder, diamond was found partly embedded in pyrope.