Do diamonds ever decrease in value?

The value of a diamond is worth whatever someone will pay you for it.

Your question reflects the vagaries of the market more than it reflects the value of diamonds.

Sometimes, people pay more for a diamond than necessary. Then when a subsequent sale is attempted, the mark-up paid in the first purchase is lost, so the seller may believe that the diamond has decreased in value.

Highest quality diamonds per se won't lose value, unless one is sold in a distress situation, in which case, the price may be less than originally paid because the seller simply 'needs the cash'.