A Very Large Unmounted "Blue Diamond"
the brilliant-cut stone stated to weigh 100ct, stated to measure 30mm x 18mm.

In my opinion it is impossible that it is a Natural 100ct 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 treated blue diamond

https://www.gia.edu/