Is there any place one can get certified and assessed, and what will it cost?

I ask because I have been offered a ring worth 450,000 THB According to the owner (who has a receipt) for a short term loan of 200,000 THB.

I just want to make sure that everything is spinning freely before I accept his ring as collateral.

The GIA in Bangkok. However you should make the borrower pay for the report.

I would loan only 25% of the appraisal on the ring. You may be getting an inflated valuation.

Ask for additional collaterals and I bet the borrower either walks away, is offended or both.

Thais are not sentimental about jewelry in my opinion and rarely are interested in diamond.

If it's worth so much I would expect them to just sell it and take the small loss.

I think you are being set up.

There are several labs in the Jewellery Trade Centre on Silom road next to the Holiday Inn. These labs will evaluate the stone, but if the stone is set in a ring it will be more difficult to asses. The cost will vary depending on the type of report you want i.e. the depth of evaluation and reputation of the lab.

However the labs will not value the stone. Diamonds can be valued (trade price) according their quality using Rapaport tables, Other gemstones may have price guides but their price will generally depend on how much a buyer is prepared to pay you for it i.e governed by supply and demand.

There is a huge difference in retail and trade prices for jewellery. If you find a jewellery shop that is prepared to put a value on your ring ensure that it is the trade price they are quoting. And even at that trade price you may have trouble finding a dealer to buy it.