Thank you for sharing your knowledge.
OK, I have emailed them here with all the details I have.
https://www.customs.go.th/
I don't have an official valuation, so the value is a guess. When I tried the calculator you recommended and entered a $5000 value, I got an 11,000 THB tariff, which seems OK. I'm dealing with jewelry of unknown value. I doubt customs can judge the value. It looks old and dirty. A friend of the family said $10k, but it was not a serious appraisal, just an eyeball figure, probably due to the one diamond.
Thanks.