Valuation
An independent written assessment of what a piece is worth, usually for insurance rather than for sale.
A valuation describes the piece, states metal purity and weight, identifies stones and pearls, and gives a replacement figure.
Insurance valuations are written at replacement cost, which is higher than what you would be paid selling the piece.
For pearls, a useful valuation records type, origin, size, shape, lustre and whether colour is treated, since those are what a replacement would have to match.
When you are buying
Have anything above a few tens of thousands valued and photographed for your own records. It costs little, and it is the difference between an insurance claim being paid and being argued about.
Resale Value
What a piece fetches when sold on. For most jewellery it is the metal value, with the craftsmanship written off.
Certificate of Authenticity
A document stating what a pearl is. Useful in proportion to who issued it and what it actually tests.
Assay
The laboratory test that establishes how much gold is actually in a piece.
Buyback
What a jeweller will pay to take a piece back. Almost always based on metal content alone.