Seems pretty simple to me:
https://www.google.com/#output=searc...w=1366&bih=626
Rare: "not ocurring very often".
By itself it is an "inconclusive conclusion" which someone has drawn based on other evidence which they don't present. This definition itself implies relativity and subjectivity. Often depends on your definition of the "time frame" or relative to other items similiar in nature.
For you to call something rare you must also, give the reason to what you are comparing it to, to draw that conclusion. It is rare in comparison to X.
You can call shit rare, all you want....it means nothing without the conclusion to the premise.
Kevin