Here's Why You Never Want To Eat A Fresh Raw Olive
Imagine yourself in Tuscany, strolling through a field dotted with Cypress and olive trees. The sun is shining, a light breeze is blowing, and you're completely swept up in the beautiful golden Tuscan afternoon. You approach an olive tree and notice that its branches are filled with pristine green olives, so you pluck one off and pop it into your mouth... only to promptly start gagging, coughing, and spitting, desperate for a glass of acqua minerale. Congratulations, you've just made one of the classic mistakes of Italian (or Mediterranean, in general) tourism: Never, ever, eat a fresh raw olive.
There's a reason why you never see fresh olives in the supermarket: They're inedible. Even though they grow on trees and look like fresh fruit, they're full of a phenolic compound called oleuropein, which is very, very bitter. The only people you'll ever see eating raw olives are growers who have desensitized themselves after years of experience (and even they usually only sample the fully-ripened black ones, which contain less oleuropein). In order to make an olive palatable, it must either be soaked in lye or brine or packed in salt; this treatment will draw out the oleuropein.
So don't make the amateur mistake and eat a raw olive, no matter how tempting it looks hanging from that tree.