Diamonds were worn by aristocratic families to ward off the plague during the Middle Ages. The poorest people always died first, since they lived closer to the docks, where the ships often brought the plague from other countries. The rich had an idea that since the poor went first, that displaying their wealth (diamonds) would keep them from infection. This just goes to show how naive people were about what really caused the plague.
Crysallos literally meant "frozen ice."
One such diamond was called "Moon of the Mountain." This gem was last seen amoung Russian crown jewels and then it disappeared in the 19th century. Catherine the Great was the first Russian ruler to possess the diamond. When it disappeared this diamond was thought to have carried its curse to its new owner.
The first recorded owner of the "Moon of the Mountain" was Nadir Shah, the Turkoman ruler of Persia (in 1739). Nadir Shah left a pyramid of skulls in his wake. His soldier's raped and pillaged each village they conquered. The "Moon of the Mountain" had its sister diamond called :Sun of the Sea." These stones traveled from Persia to Basra to the Tigris and Baghdad.
Grimaldi, David A. Amber Window to the Past New York: Harry N. Abrams, Inc., 1996.
Kendall, Leo P. Diamonds, Famous and Fatal. Fort Lee, N.J.: Barricade Books, 2001.