" ‘And he gave me these diamonds,' she said, unclasping the necklace and tossing it on the couch, where it blazed and glittered like a phosphorescent wake. 'They were his mother's, and he had them reset. The big one in the middle is called the Begum. I suppose it is disgraceful to admit that they had an influence on me, but they did. Perhaps most women like diamonds.' "
The Fortune of War. Ch 6.
(The necklace) was a splendid bauble, so splendid that its central stone had a name, the Nabob or the Mogul or something of that kind;… suddenly the name of Diana's came to mind: it was the Blue Peter, a pear shaped stone of a most surprising colour, like a pale, pale sapphire but with much more life and fire. An impious sailor had taken it from a temple in the time of Aurangzeb and it had kept the name he gave it ever since, a name that Stephen particularly liked, for not only had it a fine round sound, but it was also that of one of the few flags he could recognise with certainty, the flag that ships flew when they were about to set sail, and it had the pleasing associations of fresh departure, new regions, new creatures of the world, new lives, perhaps new life.
The Surgeon's Mate Ch 3.
" 'You are much attached to those diamonds, Villiers,' [Stephen] said kindly.
'Yes, I am. I truly love them,' she said. 'Above all the Blue Peter.' She detached the pendant stone and put it into his hand, where it lay, strangely heavy, sending out countless prismatic flashes at the slightest movement. 'I don't give a damn where they come from,' she went on, raising her chin. 'I love them passionately. I should not part with them for anything on earth and I shall certainly be buried in them. You will remember that, Stephen? If things don't go well in the autumn, I am to be buried in them. I may rely on you?'
‘Certainly you may.’ "
The Surgeon's Mate Ch 5
"Stephen looked secretly at the stone again: he had rarely seen so true an azure; and the gold rim echoed the golden specks within the stone quite admirably. But a most unwelcome comparison welled up in his mind.
Diana had possessed an extraordinary blue diamond - she was buried with it - a blue of an entirely different nature, of course, but he felt the familiar chill grip him, the sort of frigid indifference to virtually everything; and he welcomed the opening door."
The Hundred Days Ch 7
As we see from the above, when we first meet the riviere of diamonds which Johnson gave Diana the large central stone had an Indian name, but by the next book POB had rechristened it to the much more appropriate Blue Peter. It is appropriate in another way in that the word 'peter' means stone, so quite literally the Blue Peter is 'the Blue Stone'.
Diana has always been able to rely on Stephen - even to honouring her age old wish to be buried with her diamonds...