Please can I help with a little background information?
Under English law, the general rule is that the engagement ring belongs to the fiancee as soon as it is given to her, even if the marriage then does not take place!! If the engagement is broken, a true 'lady' (in the sense of manners and breeding) returns it to the young man, but in English law, the ring is hers to keep.
There is one proviso, if the ring is an 'heirloom' ring, there is a possibility that the young man might be able to convince a court that there was an 'implied' term that the ring was only to be retained if the marriage went ahead etc. / did not end in divorce etc. But believe you me, it was previously VERY hard to establish this.
The situation might be altered in a pre-nuptual agreement if the ring is specifically mentioned, but even then, the court may not necessarily have to follow this term of the pre-nuptual agrement. However, we have been told in The Times that BP has stated that William and Catherine do NOT have a 'pre-nup'.
As I understood the terms of Charles and Diana's Divorce, it was part of the agreement that Diana was still to be regarded as a member of the BRF and would continue to receive invitations to important state occasions etc. One of the terms of the agreement was that she would also retain 'for life' the 'non-personal /state-type' of jewellery she had been given [It's hard to know what to call it - it is not strictly 'Crown' jewellery but it is that category of jewellery which basically belongs to the 'BRF' rather than the individual. Note how many countries give presents of jewellery to visiting royals: if it is given to someone other than the Queen, it is generally regarded as NOT a personal present to the royal recipient concerned: if it was regarded as property, then it would be TAXABLE! I remember when Camilla was spotted wearing some lovely jewellery - it emerged that it was a gift from an Arab Crowned Head that Charles and Camilla had visited: and BP then had to announce that it was not 'Camilla's to keep.
Regarding William: yes, he is not 'Rich Rich Rich', but he and Harry are quite exceptionally wealthy young men and can easily afford to buy very expensive 'statement pieces' for their wives [in due course in Harry's case!]. By way of comparison, could I mention Prince Andrew, who is on paper much less well off than William and Harry [who received money from their mother and also, in Harry's case, a legacy from the Queen Mother]. Prince Andrew bought Sarah quite a lot of personal jewellery - from Cartier and Garrard and the like. This was 'hers to keep'; in fact Sarah 'did better' than Diana in one respect, as the Princess had a lot of 'heirloom' jewellery as well as 'official presents' jewellery.
Re the 'Spencer Family Tiara'. Regardless of what Paul Burrell may have said [I never read the book], 'Family Tiaras' such as the Spencer Tiara ALWAYS remain the property of the Aristocratic Family Male line [they are part of the 'entail' and pass to the heir along with the house etc etc]. Diana might have retained the physical use of the Spencer tiara, not under any sense of true legal physical custody, but [I presume] because she would have the most occasion to use it. In practice, since harldy anyone except the Queen gives huge 'white tie' parties any more etc etc, it is only State occasions and Royal parties when a tiara could possibly be worn [apart from by the bride at a marriage]. There would have been precious few occasions when Earl Spencer's consort would have been able to wear a tiara.
Hope some of this helps,
Alex
As to whether Catherine is a 'jewellery person' - well, here's a thought! I was looking back at some of my old 'royal information' at the weekend, and some of it was pre-the wedding of Charles and Camilla. The Broadsheet papers were continually carrying well-sourced reports saying that 'Camilla is NOT intersted in Clothes [she has only one ballgown - an old taffeta number]. Camilla is NOT interested in spending loads of time at the hairdresser etc as she is a 'hunting, shooting, fishing' type of girl. Camilla is NOt interested in Jewellery etc etc.
In other words, pre-marriage she was being painted as a 'no-nonsense Princess Royal-type of person'
Yet look at what has happened:
Camilla's hair is very blonde, from contant visits to the hairdresser. She wears los of designer clothers. She has a substantial collection of personal jewellery from Prince Charles. In short, Camilla has not remained the 'Princess Royal type of person'.
Catherine may be doing the 'Hight Street Fashion Princess' routine at the moment but I get the impression that we will be seeing her 'sparkle' in the not too distant future.
Got to rush off now to Chelsea Flower Show for the Preview day and some serious 'Royal Spotting'
Hope some of this helps.
Alex
Interesting thoughts about her first jewelry gifts coming from 'The Diana Collection.' I really wish we knew more about what, specifically, PW and PH own/inherited and what went back to the Queen so we could speculate in a little more detail. 
