I have some vague memory of the saphire being significant.
To Google I go...
Nope:
About Princess Diana's Engagement Ring
Prince Charles initially proposed without an engagement ring, insisting that Diana consider the implications of the role of his wife - being a couple under constant camera barrage and the pressure to provide the next generation's heir - before accepting. Diana, however, needed little encouragement, and after she accepted a selection of engagement rings was presented for her consideration by Garrard Jewellers. Diana choose an oval blue sapphire engagement ring that weighed in at an astounding 18 carats and was surrounded by 14 small diamonds in an elegant cluster setting.
Unlike many celebrity engagement rings, when the public must hypothesize about an engagement ring's price and jewelers are sworn to discretion and secrecy about their client's privacy, the public knew the cost of Diana's engagement ring instantly. It was actually a piece in the regular Garrard catalog and therefore could be purchased by anyone, for the hefty price of 28,000 British pounds, or the equivalent of $65,000 (at 1981 exchange rates).
At first, the engagement ring caused a bit of a stir both among the royal family as well as the public when it was discovered that it had not been especially designed for Diana. As with any bride-to-be, however, Diana's preference overruled those concerns, and the rich blue sapphire became an elegant symbol of the couple's commitment.