You can spend as much or as little on a promise ring. If she loves you she will love the ring. Buy one you think she would like that fits her style and I'm sure it will be worth it!
It depends on the sale
The sale price depends totally on the quality of the diamond and the metal setting. You can pay a certified gemologist to produce a certificate that describes the ring, and ask that the certificate include a current market value. Otherwise, you can pay a jeweler to give you an appraisal for the ring that you want to sell.
item sale i too dono
Your best option may be to sale your ring on the open market at Ebay. I would suggest getting an appraisal and then selling the ring.
A diamond of this size with this provenance would only be sold at an auction -- unless it was sold at a private sale. Its pre-sale auction estimate would be in the multiple millions of dollars US. It's last sale fetched US5,000,000 in 1979, and it was sold again -- privately -- for an undisclosed sum. You can read more below, about large diamond auction prices.
What you have to sell is worth whatever someone will pay you for it. In October 2012, there were diamonds of about this weight sold at the Christie's sale. You can read more about the sales, below. In particular a diamond weighing just over 50 carats sold for US$9,500,000. In a ring, the mounting would be an additional cost.
It would cost about $700,000 to buy Albania.
you would get about $150.00 used
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The 929 mark on a ring indicates that the ring is 92% silver. This helps to indicate the value of the product for sale.
it would cost 15 billion dollars XD