By Bill Shelton
If we consider a crystallized well-formed example of any mineral in comparison to the bulk of the crust, it will be infinitesimally small in terms of percent. Hence, it could be characterized as rare. A mineral collector often has a different slant on things and may refer to rare species or rare habits; also rare from a certain locality.
RARE: May mean distinctive or uncommon as it applies to minerals. Similar to infrequent; also may imply choice such as the best piece.
Amongst various factors affecting rarity and our perception of rarity consider the following possibilities. A rare element may greatly restrict how much of a certain mineral exists – then consider how much might actually be identified and recovered. Then, what quantity gets into the “mineral marketplace” and ultimately becomes available to you, the collector. Further, will you be aware of the few dealers who may have this “rarity”? Minerals that may be considered rare on the basis of chemistry might include minerals found to contain certain rare elements, perhaps in significant quantities as seen in their formulas. Certain rare elements, however, have quite a few commonly found minerals readily available for sale. Rubidium, #22 in rarity, has essentially no mineral examples where it is present in much quantity because it tends to be dispersed in low amounts in other species such as the feldspar group. Bismuth, #70 in rarity has almost 150 mineral species – a huge number for such a rare element. Bismuth can be found as a native element and in species like bismuthinite. Rarity for elements is a number from 1 to 92 based on the relative abundance in the earth’s crust. See the 2011 Gem and Mineral Almanac Section V: Elements and mineralogy for more on this topic.
Now a real factor to deal with is the mineral marketplace. If you can’t find a sample for sale perhaps it is simply unavailable rather than truly rare. Maybe there are only a few examples of the mineral known to exist. That sounds like it might be really rare. Examples exist where a single specimen is known –they certainly qualify as rare. A few minerals are only known from one or a few places – they may be rare but not necessarily. Consider charoite from Russia – essentially none exists elsewhere but a collector can buy a dozen pieces at most any major mineral show – never mind online. One locality minerals may be rare-it might be dependent on the number of pieces available for sale rather than anything else.
If location is factored in, rarity takes on a new appearance. Some minerals are almost never found at particular places. They can be described as rare from that place. A collector can go to mindat.org and get a number for each species that suggests how many places it has been recorded from. Similarly, quality can affect our perception of rarity. A number of minerals rarely produce very fine crystal groups even though they are common species. These minerals, then, are rare as fine specimens. Crystal size can also be used. Large, perfect crystals for many species can be few and far between even for minerals that are relatively common.
A gwindel of quartz may be rare from some places but collectors generally discount quartz as a rare species due to the ready availability of ordinary specimens and the fact that it is found at so many localities. Yet, a large, perfect gwindel will not be easy to find. So, when you hear that something is rare, listen for an explanation. It is my impression that “rare’ is probably overused and incorrectly applied with regard to mineral specimens. Also, bear in mind that sellers can use the concept of rarity to influence potential buyers. Texts in our field use the term rare in a relative sense. For example, Dana’s Manual of Mineralogy (1971) lists rare hydrous carbonates on page 325 and includes aurichalcite. On page 415, aegirine is said to be relatively rare. Page 471 tells the reader pollucite is a rare isometric mineral. Mindat.org lists 869 places for aurichalcite, 989 for aegirine and about 150 for pollucite. Compare these to, say, diamond with 696 localities or charoite with only 9. I would like to add that none of these is rare in the marketplace in my opinion.