Few stones inspire thoughts of summer quite like turquoise, offered in vivid hues ranging from a clear, pale blue sky to a robin’s egg tone or a deep azure that conjures images of Caribbean waters.

The lack of a grading system for the popular stone has prompted a father-son team to spend the last five years working on one. Turquoise stones from the blue and green master sets at the Turquoise ...

A 7.03-carat fancy deep blue diamond mounted on a platinum ring by London high jewelry house, Moussaieff, is the top lot at Christie’s Geneva Magnificent Jewels sale. The blue rectangular-cut gem with ...