Actually, while there is only know known stable isotope of cobalt (mw=59), the standard molecular weight of cobalt of 58.933195g·mol−1, recognizes that other isotopes are always found with cobalt in nature, albeit in very low proportions.
What's interesting, is that all isotopes of an element have the same chemical properties (because their electron shell is the same, since they have the same atomic number and hence nuclear charge), so cobalt sulfate found in nature will contain cobalt of many isotopes.
This is much like heavy water, which contains heavier isotopes of hydrogen (duterium and tritium are names for hydrogen with a mw of 2 and 3 respectively), and which can be found in all bodies of water, just in very small proportions. Interestingly enough, Isotopes of oxygen are also found in water, and the ratio of the isotopes of oxygen in ground water is being used in surveying, to give an estimate on the "age" of the water (as it relates to how quickly that groundwater is replenished from rain water), to determine the long term viability of pumping from an aquifer.
Oh, and yes, I get the joke. :)