While I agree somewhat, I highly doubt that royals assume that everyone has access to tiaras. We are seeing that many royal families have opted to use tiaras less frequently, so as not to appear ostentatious or out of touch. Tiaras are a symbol of the monarchy as well, and so it is understandable that the British monarch should choose her tiaras--and those of her family--very carefully. Her Majesty has faced criticism during her reign, as well, so every choice down to wardrobe (and especially jewels) must be made with much consideration. I doubt that she actually assumes that anyone has a collection of tiaras like her own...but at the same time, it is not really the job of the queen to know each piece in her catalogue. Care of the jewels belongs to someone else entirely, so chances are if the queen is not already familiar with a piece, she doesn't know everything about it. I don't imagine all the jewels in the royal collection are displayed in glass cases in some room where she might stop and consider them; in all likelihood they are in their respective boxes in some sort of vault, perhaps even many separate vaults. I would guess that HM considers several of her tiaras rather like signature pieces, and is saving the others less seen for members of the family yet to come. As far as specific pieces (like the Teck tiara), one can conjecture possible reasons why it has not been seen of late. My own guess is that it has a very wide base (as many tiaras of the period do), and is therefore difficult to wear with the more modern hairstyles of to-day. I believe that this was the reason that the Queen Mother rarely wore this tiara as well. However, I think that the possibility of it coming back into the spotlight within a decade or so are very good, and I do look forward to that.
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