Antique Diamonds

This may be a dumb question, but why do antique diamonds still sparkle and not look dull or cloudy? Everyone says modern diamonds need to have good color, clarity, excellent cut/polish/symmetry or they’ll be dull/milky/cloudy/yellow/dark etc. Usually antique diamonds are pretty low in color and clarity and the cuts are never symmetrical since they’re done by hand. Don’t get me wrong I love antique diamonds