We haven't been able to discover much about this type of olives. The most we've been able to find out is that they aren't cured in vinegar like other sorts of olives. They are just processed in brine, so they are slightly salty, but without that bitterness found in other olives. This also preserves more of the olives' natural minerals and sugars, so more of the natural taste (the taste that ends up in olive oil) is there.
Apparently this type of preserveation also cuts down on their shelf life, though, which may be why they're a little harder to find.
Have you ever tried Castelvetrano olives? If you find them, try them! As they say at at Wally's Wines, even people who don't like olives often love these. They are great as table olives.
• Buy Castelvetrano olives at Wally's, $7.99/package