I agree with the following thread that a diamond ring is not an investment. therefore, no capital loss could be claimed. If you tried to take a capital loss it would stand out on form 8949.
https://www.dmia.net/diamond-rings-investment/#:~:text=The%20general%20consensus%20is%20that%20diamo...
you can contribute it to charity but your deduction would be limited to fair market value which would be the same as its selling price. you would need to be able to itemize your deductions to get a tax benefit. if the FMV is more than $5,000 you need a formal appraisal which must be attached to your tax return.
in short, I don't see you getting any tax benefit from the decline in value.
