Diana's father, (Edward) John - known as 'Johnnie' - was Viscount Althorp when she was born and would become 8th Earl Spencer in 1975. He served as Equerry to both King George VI (1950–52) and to Queen Elizabeth II (1952–54) and is allegedly famous for riding his ceremonial horse a little too much in the way of The Queen in her carriage. After divorcing Diana’s mother, Frances, he famously married Raine, Countess of Dartmouth, the former wife of the 9th Earl of Dartmouth, and the daughter and only child of the romantic novelist Barbara Cartland on 14 July 1976.
The Spencer family is one of the most distinguished families in Britain. Several family members have been made knights, baronets, and peers. Hereditary titles held by the Spencers include the dukedom of Marlborough, the earldoms of Sunderland and Spencer, and the Churchill viscountcy. As well as Diana, the most prominent member of the family has to be Sir Winston Churchill, whose Coat of Arms is reproduced here. His full name was Winston Leonard Spencer-Churchill. It will become evident that the second and third Quarters are indeed the Spencer Coat of Arms.
There was no need to design any new Coat of Arms. Much of the memorabilia carried her Spinster Arms. Surprisingly, though, these were borne on a strange, almost eight-pointed shape rather than then usual Lozenge. This was because of the nature of the Four Quarters of the Arms as they would otherwise have been squashed. Furthermore, the usual blue ribbon was changed to pink in the prevailing version at the time (shown below) and the shield was surrounded by what looks like may blossom.
On her marriage, Diana simply impaled her father's Coat with that of her husband. A Sinister Supporter of a Griffin was 'invented' but that was hardly a difficult task as it was heavily based on her father's Supporters. This included, though, a Chain attached to a Coronet and reflexed over the Griffin's back. This not only reflected the Scottish Unicorn which it replaced, but was also taken to reflect Diana being "shackled" to the Royal Family. (Please note a) That the late Queen Mother's family Supporter wasn't similarly "shackled" and b) That HRH the Duchess of Cambridge is the first major Royal Bride since Diana whose family Supporter isn't "shackled" either.)
The gallery below shows the three famous versions of Diana's Coat of Arms, culminating in her post-divorce version which she didn't use as much as her cypher, full name or image. Please note the transition from the marshalled Shield of a married woman to the lozenge of a single (if divorced) woman.
I once raised the matter of her Coronet with the College of Arms. As a woman who had never been a princess in her own right but by marriage and since she had been married to the Prince of Wales at that and had carried his Coronet with one arch, why was the Coronet apparently assigned to her on her divorced a plain Coronet without any arch. It was admitted that this was either artistic licence or a mistake. This was ironic since, having never (quite) remarried, Diana remained Princess of Wales if not 'The' Princess of Wales and was ceremoniously stripped of the title Her Royal Highness.
She would, of course - even in HM The Queen's televised speech on the eve of her funeral - remain simply 'Diana'.to edit.