Upon his bloody finger he doth wear A precious ring, that lightens all the hole, Which, like a taper in some monument, Doth shine upon the dead man's earthy cheeks, And shows the ragged entrails of the pit.
Titus Andronicus, Act ii, sc. 3. First Folio, "Tragedies," p. 38, col. B, lines 53-57.
This certainly was suggested by the common belief in naturally luminous stones, a belief partly due to a superstitious explanation of the ruddy brilliancy of rubies and garnets as resulting from a hidden fire in the stone, and partly, perhaps, to the occasional observation of the phenomena of phosphorescence or fluorescence in certain precious stones.
It will have been seen that the text of Shakespeare's plays gives no evidence tending to show any greater familiarity with precious stones than could be gathered from the poetry of his day, and from his intercourse with classical scholars, such as Francis Bacon, Ben Jonson, and others of those who formed the unique assemblage wont to meet together at the old Mermaid Tavern in London. That a diamond could cost 2000 ducats (#5000), a very large sum in Shakespeare's time, is noted in one of his earliest plays, the Merchant of Venice (Act iii, sc. 1), and the following injunction emphasizes the great value of a fine diamond:
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