Genuinity of the diamond, appropriate certificates
The only way is to approach multiple sellers which will help you determine the market value
I’m not sure historical rates will be available. Precious stones don’t form part of the regulated market, so the value you see may not be the value someone else sees and hence the rates may vary.
A diamond may not be that good of an investment due to its opaque nature. I would bet on Gold and that would be my choice of investment. Apart from that useful and precious metals should do better than precious or semi-precious stones.