Legend has it that Saint Patrick gave a four-leaf clover to a group of his followers; the fourth leaf put there by God to bring luck. St. Patrick believed the first three leaves represented hope, faith, and love. While the actual probability of finding a four-leaf clover is unclear, at best, it may be 1 in 5000. Although since the gene for the fourth leaf is inheritable, if you find one, another may be close.
A four-leaf clover represents just one kind of rareness. One might find a 4-leaf clover just about anywhere. Four-leaf clovers are not just restricted to Ireland. Four-leaf clovers are rare because at any given locality they occur in very minuscule numbers.
The idea of whether rareness imparts values has tormented philosophers, including Nietzsche. “Whatever can be common always has little value. In the end it must be as it is and always has been: great things remain for the great, abysses for the profound, nuances and shudders for the refined, and, in brief, all that is rare for the rare.” But of course, Nietzsche does not define rare. What does “all that is rare for the rare even mean?” Freakin’ Nietzsche.
We all feel we know what rare means. But contrast the case of four-leaf clovers with platinum. Platinum is special for me. For my 10th wedding anniversary, I had a custom wedding ring made of platinum for my wife. This platinum band was to replace one from our youth when I had more limited income and could afford a metal less “precious” and less “rare.” Yet, platinum represents another kind rarity, occurring in great abundance but only at a few locations. Locally abundant but geographically restricted.
In a classic 1981 paper, Dr. Deborah Rabinowitz, a professor at the University of Michigan, laid out the seven forms of rarity. What makes something rare depends on three characteristics; geographic range, habitat specificity, and local population size. First, is a species found globally or only at a single location? Two, is species seen at any given site in low numbers? Third, is the species only found in a specific type of habitat?