Would abnormally high levels of cobalt on a moon cause it to appear blue to those on the planet it orbits?
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2 Answers
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Elemental cobalt is not blue
The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal.
If you want to have blue with cobalt, you need to have cobalt blue
Cobalt blue is a blue pigment made by sintering cobalt(II) oxide with alumina at 1200 °C. Chemically, cobalt blue pigment is cobalt(II) oxide-aluminium oxide, or cobalt(II) aluminate, $CoAl_2O_4$. Cobalt blue is lighter and less intense than the (iron-cyanide based) pigment Prussian blue.
Prussian blue is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula $Fe^{III}_4[Fe^{II}(CN)_6]_3$
Answering your question, if you have abnormal levels of cobalt alone you won't get any blue. If you have the right cobalt compounds you might end with a blue looking moon.
I am not sure about their stability in space conditions, though.
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2$\begingroup$ There are plenty of aluminum oxides on our moon. I am sure cobalt would join the party if there were some up there. Without a dream in its heart. Without a love of its own. $\endgroup$– WillkNov 7, 2019 at 13:19
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Cobalt occurring naturally does not appear blue.
You can see from this picture of cobalt ore as well as three of the most common naturally-occurring cobalt sources on Earth, cobaltite, glaucodot, and skutterudite that none of them appear blue.
