Its a serious question, honest.
Compare the bright blue waters of the Carribean, for example. I thought the
blue was supposed to be due to the blue sky. Yet the waters around Thailand
are vibrant emerald green. Its not algae or anything like that - the waters
a crystal clear. And neither is it the underlying rocks or corals: the
emerald color is just as pronounced even over sandy white Thai beaches.
Presumably its not the green sky doing it ;-) So what is it? Is the water
really so rich in some mineral or other that turns it green? If it *is* a
mineral, then what is it?