So, if the Queen doesn't wear her crown for formal occasions - even something as formal as opening Parliamentary business - when does she ever wear it at all? The answer is only indoors, and only after 6pm. Six o'clock in the Royal household, or at a society event where the Royals are present, is when the ladies change into evening wear. The Queen may choose to wear a crown (although probably not, as they're surprisingly heavy), or a tiara. Many of the ladies present may also wear a tiara.
Evening time is also when any recognized and desirable family jewels may make an appearance, including the crown jewels. For obvious reasons, those who own such valuable assets prefer not to wear them outdoors, where it would easier for them to be damaged, lost or stolen. Always pack two outfits if you're going to a royal event; if you're lucky enough to be asked to stay the night, you'll be expected to step your attire up a gear!