Whatever someone will pay for it. We can look up similar diamonds on-line but we only have 2 of the 4 C’s (Carat and Color). The other big criteria are Clarity and Cut (how much and how well it needs to be cut in this case). That is big diamond but most people want traditional diamonds from a reputable jeweler. The diamond was may have difficult to see flaws or a less than perfect cut that decrease its value as well. The color isn’t the most popular either.
The only way to know is to have it officially appraised and then find a buyer for it.
Fair enough. But in general terms, is this guy set or does he just have a big hard rock. Best/worst case scenarios?
Big hard rock. Few hundred bucks. Similar story happened in the 90’s, and the guy thought is was worth millions. The color absolutely made it worth a few hundred as a novelty.