Just because mom told you her old diamond ring was valuable, it means it is. It might not be a real diamond to begin with. So, before you rush selling it, get a definite knowledge of its authenticity and quality.
A qualified appraiser, one that is not into selling or buying diamonds (peerdiamonds.com), can give you their unbiased opinion of the its characteristics and condition. They can even highlight negative and positive attributes that define its value.
Having your own unrealistic price expectations is the fastest way to disappointment you. There are approaches to determining a more reasonable price. First, on your appraisal, ask how much the diamond might worth in certain market. A reputable appraiser follows current market trends and they might help you understand the probable resale value of your diamond jewelry.