Fair Market Value
What a
willing seller will take from a willing buyer for an item in a second hand market. (ex.
online auction)
The
amount paid by a consumer on a new item in a retail environment. (ex. retail store)
The most
an insurance company will pay to replace an item in case of a loss with inflation taken
into consideration. (ex. Insurance appraisal)
There are other markets for assessing values for jewelry.
However, these "markets" are what we typically deal with on a daily basis.