In a forthcoming American Minerologist paper, Hazen and Ausubel outlined a new mineral-classification system to help geologists better understand the designation of “rare.” They based their work on a similar system by the biologist Deborah Rabinowitz, who studied rare biological species.

According to Rabinovitz, a species can be considered rare if it meets at least one of three criteria: a small geographic range, highly specific habitat requirements, or a small population size.

Writing in The Atlantic, Dr. Alex Berezow has all of the fascinating details.