I would be very surprised to see Meghan borrow from jewellers. My sense is Meghan's "royal" jewel box will comprise jewels from:
> Harry's share of Diana's personal jewels
> Presents from HM and the PoW
> Presents received at their wedding, and ones, if any, they may receive whilst carrying out engagements and trips overseas, which will technically belong to the Royal Collection
> Any jewels they choose to buy themselves.
Further, HM may, from time to time, make some loans of jewellery to her, quite like Sophie Wessex, but these are unlikely to be significant or pieces with major historical provenance. Similarly, Camilla may loan some jewels that she does not use to Meghan, as I suspect she did with the diamond earrings.