Fun fact about lapis lazuli/ultramarine: if you grind it too finely, it turns gray. A friend who has recreated many medieval paint pigments discovered this the hard way... Just another reason it is so complicated to produce.
The reason is that the crystalline structures that finely select for that narrow band of blue are destroyed. The same thing happens if you put an oil drop on water: at first brilliant rainbow bands of color are produced (from the selective reflections off either side of the oil, where the oil thickness is a multiple of the light quarter-wavelength). Then the oil spreads further, until it is less thick than a blue light quarter-wavelength, and it turns dark.
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