Hallmarks are usually stamped on the item — 585, 750 or 999. If you cannot see it, pick the closest match and the partner will verify on-site.
Common 14k = 585, 18k = 750, 24k = 999.
No problem — pick this and the partner confirms the purity on-site. Your estimate stays indicative.
If you do not have an exact weight, an approximate value is fine.
You can stop at any time — your data is not held in the open.