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Scarcity in is one issue that determines value in fossils and minerals. Other factors are: popularity, looks and history. If a specimen is rare but doesn't have any eye appeal, it's value can be greatly affected. We have some greenland sodalite that fluoresces bright orange under a black light. It's rare and not very well known, but it has great eye appeal so people buy it when they see it, despite the high cost. I intend to elaborate more on this subject with a full article in the near future. Keep an eye out.