The eye stems back as far as the 6th century BC, most commonly found in the Middle-East and Mediterranean (Greece and Turkey), where it was used as a good-luck, or protection charm. The eye has become used for commercial purposes in modern Greece, while it is still a widespread belief in Turkey. Alternatively, it was said to cause injury or harm to those that the eye is directed at! Another source tells me that those with blue eyes were more prone to cast an evil eye on someone – hence the common blue colour choice!
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[ Now you're officially educated on the subject, shop for your own evil eye! I'll start you off with some of my faves... ]