The source for this statement, note number 22, is a book about England's queens-consort published in 2008. Here's the relevant passage:
Notice that the author doesn't cite a source for this allegation. (Incidentally, the 'magnificent 18,000-pound dowry' is wrong; although Philip IV did originally discuss giving his daughter a dowry of 18,000 livres tournois or 'pounds of Tours', the equivalent of 4,500 pounds sterling, he ultimately decided that her 'dowry' would be the duchy of Aquitaine, over which he and Edward I had battled for many years.) Agnes Strickland in her Lives of the Queens of England, volume 2, p. 131, published in the middle of the nineteenth century, first invented the notion that Edward II gave jewels to Piers Gaveston which Isabella considered rightfully hers. The passage is below. Notice that Strickland doesn't claim that Gaveston actually wore Isabella's jewels in front of her or went 'peacocking about' in them; that appears to be a fictional invention that appears in some twentieth-century novels about Edward II and Isabella of France which has found its way into non-fiction.
Yet even here, Strickland doesn't claim that the gifts were Isabella's, only that she 'resented' her husband giving his own gifts to Gaveston and was displeased about items she considered her own 'heirlooms' being given to someone else. Later writers picked up her claims about Isabella's resentment and displeasure - for which she didn't cite any source - and ran with them, until eventually we reach the whole nonsense about Gaveston prancing around in jewels which Edward has bestowed on him and which actually belonged to Isabella. The source for this entire story is the Annales Paulini, which states: "The king of France gave to his son-in-law the king of England a ring of his kingdom, the most beautiful bed ever seen, select war-horses, and many other extravagant gifts. All of which the king of England straight away sent to Piers [Gaveston]." As I've pointed out before, this in no way states that Philip gave these gifts to Isabella, or even to Isabella and Edward jointly. They were Edward's own wedding gifts from his new father-in-law, and why would Philip IV give his daughter war-horses anyway? Philip 'gave' the items to Edward, and Edward 'sent' them to Gaveston, his regent of England while he was in France to marry Isabella. There's no real indication that Gaveston was meant to keep the items permanently, and Isabella is not even mentioned. Why would she consider these items her own 'heirlooms'? The gifts were given to Edward II. By the norms of the fourteenth century, anything given to Edward legally belonged to Edward. Anything given to Isabella legally belonged, also, to Edward. When Isabella wanted to make her will when heavily pregnant in October 1312, as a married woman she required her husband's permission to dispose of her goods, as legally everything she owned was his. In a fourteenth-century context, to suggest that Isabella would have considered items given to her husband to be her own, to be 'part of her dower', makes no sense. And only one of the gifts given to Edward by Philip was jewellery anyway, the ring, unless perhaps the other 'extravagant gifts' included other pieces of jewellery. So where on earth has this nonsense about Gaveston 'peacocking about in Isabella's jewellery' even come from, and why are books of allegedly serious non-fiction repeating it as fact?
Agnes Strickland's books, and her prejudices, have proved extraordinarily influential and enduring. She often cites the St Albans chronicler Thomas Walsingham as a source for Edward II and Isabella, as though a man who was born c. the 1340s and died in c. 1422, the year Edward and Isabella's great-great-great-grandson Henry VI succeeded to the throne, is in any way a reliable source for things that happened in 1308. The letter Isabella supposedly sent to her father complaining that she was 'wretched' comes straight from Walsingham. Strickland, however, living and writing in the Victorian era - she was born in the late eighteenth century - can be forgiven for her prejudices and for her misreading or misunderstanding of fourteenth-century sources in Latin. For her time, she did incredible research and her contribution to medieval scholarship was immense, and we should acknowledge her as a pioneering female historian. Modern writers who copy her prejudices and her misconceptions, however, and who don't check the primary sources for themselves, and repeat stories told in novels as though they're fact, cannot be so easily forgiven.
Also, Edward did not 'refuse' to grant Isabella her own lands: he gave her his county of Ponthieu on 14 May 1308, three months and seven days after her arrival in England. This is another modern myth, created - like numerous other modern myths - by a writer determined to find examples of Isabella's alleged suffering and victimisation at the hands of her husband everywhere. The usual dower lands given to the queen of England were still held by Edward's stepmother Queen Marguerite, and alternative arrangements had to be made. The widow of Edward's nephew the earl of Gloucester, killed at Bannockburn on 24 June 1314, did not receive her dower lands until mid-December 1314 nearly six months later, and in 1317 it took almost another six months to partition Gloucester's lands for his three sisters. Philippa of Hainault married Edward III in January 1328, and received no lands for more than two years, until February 1330. And Edward II gave Isabella a household of almost 200 people, far larger than that of any previous queen of England, so I haven't the faintest idea where this 'refused to give her a household' comes from. But who cares about silly things like facts and about being fair when you can further the Victim!Isabella agenda?