A Large Unmounted "Blue Diamond"
the brilliant-cut stone stated to weigh 70ct, stated to measure 29mm x 17mm.

In my opinion it is impossible that it is a Natural 70ct Vivid Blue Diamond as if it was it would be most important blue diamond ever sold at auction with valuation of up to $100 million.. If it is in fact a diamond at all whether treated color, once again it is impossible to sell without first obtaining a GIA gem diamond certificate to prove that it's natural and the quality as per the 4 C's.

Happy to review values once a GIA certificate is obtained.

Valuation is based on being a treat blue diamond

https://www.gia.edu/