The Jeweler's Jumble

A young man, seeking a diamond for his beloved, visits Jeffrey the Jeweler and asks for his heaviest diamond.

Jeffery has four spectacular diamonds. One is lightest. Two are in the middle and weigh the same. One is heaviest. Unfortunately, Jeffery is somewhat disorganized and absent-minded, and has forgotten which gem is which.

Explain how Jeffrey can determine which diamond is which, using a simple balance scale with a minimum number of weighings.

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