I think the topic of the Spencer tiara is relevant. It is one of the most classic and instantly recognizable tiaras in the world.
I am surprised to read (above) that the tiara actually belonged to Charles. Is that true? I thought it belonged to the Spencers and there's another thread here on TRF where people are discussing Diana's brother's rudeness in demanding it back (as she had other tiaras to wear and his wife did not).
I had always heard that story as true - but it really belonged to Charles?
On several blogs it is said to be assembled from several pieces of jewelry belonging to the Spencer and Montague families, although no one can verify that it's really 18th century (probably not).
At any rate, I thought most people here had ruled it out because it belongs to the Spencers and wouldn't make sense for Kate to wear.
News story today (AP is one source) says it's the Fringe tiara:
The Queen's*Fringe tiara tipped to be Kate's headpi | Latest celebrity news hellomagazine.com
I must say that I can picture the Fringe on Kate.