Cobalt, like many of the transition elements, can take on a number of oxidation states and changing oxidation states are often associated with color changes.1 On other occasions, the oxidation state may not change, but the amount of water of hydration in crystals of these compounds may lead to color changes.
The little red 'gems' in the above image are crystals of cobalt (II) sulfate heptahydrate (CoSO4.7H2O), which you may have guessed has a oxidation state of 2+ and seven waters of hydration. Apart from being rather pretty at ~4x magnification with oblique lighting, cobalt provides a useful and often unthought about about role in many biology and chem labs. Here, I've used the sulfate because it was available for imaging, but the closely related hydrated cobalt (II) chloride hexahydrate(CoCl2.6H2O) is also reddish, albeit a bit closer to pink than the brick/ruby red I associate with the sulfate.
What makes the chloride special is that it's dirt cheap, fairly innocuous, and easy to dehydrate/rehydrate. That last part is particularly useful because the anhydrous form (CoCl2) is a rather pleasant shade of blue which is very visually distinctive from the pink hydrated form. We use this in the lab all the time in desiccators, which is just a fancy word for a container which through some method keeps dry stuff dry. The anhydrous chloride, often mixed in with the desiccant, acts like a canary in a coal mine. If the desiccator stops working and moisture starts creeping in, the chloride will hydrate and turn pink, acting as a warning.2
Going back to the sulfate, the kid (or secret agent) in all of us would be pleased to know that apart from uses in Serious Science Stuff, it can also be used as an invisible ink.3 I have not written anything here in it, or at least, as far as you can tell
Posted as part of the #sciencepic challenge.