Sharon Osbourne's very grateful a fan returned her massive diamond ring ... and, it clearly pays to be honest -- 'cause she's offering that fan a once-in-a-lifetime experience.
Here's the deal ... Osbourne revealed to her Instagram followers she lost her wedding ring at a Birmingham event for soccer team Aston Villa's new store -- but, luckily, a Good Samaritan found it and chased her down in the parking lot to return it.
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At the time, Sharon explains, she was so shocked, she didn't get the fan's name .