"In the study, appearing in the journal Revue de Gemmologie, Farges' team recount how, in December 2007, they found a lead model of the Blue Diamond in the archives of the National Museum of Natural History. The model was made by a Paris jeweller, Charles Achard, whose clients included a "Mr. Hoppe of London." Using measurements provided by the Smithsonian, the French team used a computer to see how the gems match up, and find that the Hope Diamond fits perfectly inside the Blue Diamond."
I haven't seen the gemolgy journal referred to. The whole article is sketchy and hard to follow. Just why the lead model was made isn’t clear to me. It appears to be of the French Blue prior to its theft and recutting but even that is open to interpretation. I'll try to find out more.
The thing could be a zircon or paste as far as my knowledge of diamonds goes.