For me it not only depends on the financial value of the piece but the sentimental value too.
For example, I have my great grandmothers Mikimoto pearl strand and matching earrings and my sister asked to borrow the earrings for a contest she was in. I allowed her to borrow them, however she's competing again and this time she's going to be in Brazil for the competition, so no way is she borrowing those earrings. Not only would they be difficult to replace, because they're antique and match the strand perfectly but they are also something I'm very sentimental about. This is the only whole piece of my great grandmothers jewelry that I have and I'd be really upset if a part of the collection was lost.
Most of the pieces that I give great care to hold some part of my family history. Another good example is my opal necklace, which used to be an opal ring that my grandad gave to nan (unfortunately I never met him). I had it reset into a necklace for my 18th birthday using the opal , the gold from the original ring and some gold and diamonds from my great grandmothers wedding ring. So once again a very sentimental piece, which is always stored carefully and no one is allowed to borrow (I'd probably lend it to mum though if she asked).
For me I think it's more about the sentimental value of my jewelry than it's actual value.