Do you have paperwork regarding the diamond? If not, it would be to your advantage to get certification ie a GIA report.
Selling a pre loved diamond ring as a private seller is difficult.
There are a few free sites like Loupe Troop. There is also eBay (their fees about 13%). There is also Auction houses but the sellers premium is around 20% (and there is also a buyer premium too) or on consignment. Consignment can be up to 50% of sale price.
Once you have the diamonds specifications you can check online for comparable diamonds to estimate value. Keep in mind, as a private seller you can realistically only expect to get about 50% to 70% of on line price. The setting is basically gold scrap value. Ignore any valuation for insurance values, they are not anywhere close to what you can sell for in the secondary market.