The huge majority of the jewellery we see the Queen wearing in public is her personal property. A small portion of those pieces are heirlooms of the crown. (None of it is Royal Collection - though some blurring is beginning over wording used for the Diamond Diadem, crowns, regalia, and so on - not the topic of this thread.)
The Queen lends personal pieces to royal ladies she chooses to lend to. Past practice has been for personal property (of that sort) to pass on tax free if it passes straight from monarch to monarch. That keeps the everyday working jewels of the reigning line of the family intact as a workaday collection of jewellery. (Pardon my simple language here.) To keep the personal collection intact (for the reigning line) pieces are loaned and not given.