From the Greek ops (the eye). Considered unlucky for the same reason that peacocks' feathers in a house are said to be unlucky. A peacock's feather, being full of eyes, act as spies in a house, prying into one's privacy. Similarly, it is unlucky to introduce the eye-stone or opal into a house, because it will interfere with the sanctity of domestic privacy. (See Ceraunium).
Not an opal Wrapped in a bay-leaf in my left fist, To charm their eyes with.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894