The visual appearance or optical properties of a mineral or crystal, especially as seen through a microscope.
From Greek 'di-' (two) + 'opsis' (vision or appearance), originally referring to the two-fold appearance of diopside but extended to describe visual/optical analysis generally.
Ancient Greeks loved making words from 'opsis' (seeing)—that's where we get 'synopsis' (seeing it all together) and 'myopia' (short seeing), so diopsis literally means the mineral shows you two different sights depending on angle.
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