Plural of diopside, a type of green or brown mineral belonging to the pyroxene family that is commonly found in igneous and metamorphic rocks.
From Greek 'di-' (two) + 'opsis' (appearance), named because it has two different optical appearances depending on viewing angle. The term was coined in the 19th century when mineralogists were systematizing crystal specimens.
The name diopside is a perfect example of how scientists use Greek to describe what they literally see—under a microscope, this mineral looks like two different things depending on how you turn it, so they just said 'two appearances' and that became its name forever.
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