The Annales Paulini (ed. Stubbs, p. 258), written in the 1330s, describe something which took place shortly after Edward II's wedding to Isabella of France in Boulogne on 25 January 1308:
Rex Franciae dedit regi Angliae genero suo annulum regni sui, cubile suum quam pulcrum oculis non vidit aliud, destrarios electos et alia donaria multa nimis. Quae omnia rex Angliae concito Petro misit.
Translation: "The king of France gave to his son-in-law the king of England a ring of his kingdom, the most beautiful bed (or couch) ever seen, select war-horses, and many other extravagant gifts. All of which the king of England straight away sent to Piers [Gaveston]."
This is the sole and entire basis for the often-repeated modern story that Edward II cruelly and heartlessly gave away his new wife Isabella's jewels and/or wedding gifts to Piers Gaveston. Hmmmm, curious. Do you see Isabella's name mentioned anywhere here? No. Are the gifts from Philip IV said to have been given to her? No. Are the gifts said to have been given to her and Edward jointly? No. It says perfectly clearly that "the king of France gave to his son-in-law the king of England..." not "to his son-in-law and his daughter...". Does it say that Edward intended Piers to keep the items permanently? No. Does it say that Edward allowed Piers to keep and wear Isabella's jewels, or keep and use gifts given to her? No. Does it say that Edward allowed Piers to wear Isabella's jewels in front of her? No. Does the passage actually say anything about jewels at all, apart from one ring? No. Does the passage actually say anything about Isabella at all? No. Are we supposed to think that Isabella's father would have given her war-horses? Errrmmmm.
The Annales Paulini (written over two decades later) merely say that Philip IV 'gave' (dedit) some gifts to his new son-in-law after the wedding on 25 January 1308, and that Edward II 'sent' (misit) them to Piers Gaveston, i.e., Edward sent them from France to England. It is entirely possible that Edward merely intended Piers to store the gifts for him safely, Piers being regent of England during Edward's absence, and being of course the person Edward trusted most. And even if Edward did intend Piers to keep the gifts permanently, they were his possessions now, no-one else's, and he could do what he liked with them. Giving them to Piers to keep, if he did, was tactless and rude and it is possible that Edward deliberately intended to annoy Philip IV by doing so, but there is absolutely no reason to suppose that he also in any way intended to offend or hurt Isabella. To whom, let it be pointed out again, the gifts did not belong. The Annales say that Philip dedit/gave the gifts to Edward, who misit/sent them to Piers. The word dedit is not repeated, as we might expect to find if Edward had given the items to Piers with the intention that he should keep them. Philip gave his daughter a magnificent trousseau to take with her to England, a list of which still exists, and it is clear that the gifts mentioned in the Annales were for Edward alone. Obviously, Edward, being in France, would have to take or send his gifts back to England at some point. Why should he not send them to his regent?
To sum up: does this passage say anywhere at all or even hint that Edward II gave his queen's jewels away to Piers Gaveston? Nope, not even close. Does it say that he gave away his and her wedding gifts to Piers Gaveston? No, it only says that he sent the gifts given solely to him to Piers, which may simply mean that he intended Piers to store them safely for him. So why then are modern books, fiction and so-called non-fiction, and online articles so full of breathless claims that 'Edward II gave away his wife's jewels/gifts to his lover!'? Why can't people do some proper research and read the source they're supposedly quoting? Why do they insist on repeating a story without bothering to check that it's true? Someone once saw this passage in the Annales, put two and two together and made 857, and it's been repeated as 'fact' ever since, with numerous details embroidered to make Edward look even worse and to add to the popular modern Victim!Isabella myth. As Anerje pointed out in a comment here, 'Edward II gave Piers Gaveston Isabella's war-horses!' doesn't quite have the same indignant ring to it, but would at least be more accurate, given what is actually written in the Annales Paulini.
The story recounted here a quarter of a century later is simply that Edward II, newly married in Boulogne, sent the wedding gifts from his new father-in-law to his regent and best friend in England, most probably just to store for him, and from this basic and rather dull fact, writers from the nineteenth century onwards have made up a load of overwrought nonsense about the king deliberately hurting his new wife by giving her own jewels and gifts away to his lover, with some fiction piling melodramatic insult on injury by having Piers flaunt himself in the jewels in front of a distraught Isabella. Poor Edward, to be maligned like this, to have such silly stories made up about him and repeated endlessly as 'fact'.