One of the two principal antique engagement ring trade routes passed through Venice. It enjoyed a monopoly of the ideal cut diamonds trade on its way to the main towns of southern Germany up to its final destination in Bruges.
When antique engagement ring is cut too deep light leaks out the bottom, brilliance is lost, and the ideal cut diamonds will appear to be dark.
Most diamond were formed more than 100 miles below the surface of the Earth, some from perhaps 400 miles down.