You will get better advice posting in the colored gems forum.
Unfortunately it's impossible to evaluate a stone from a photograph. Key questions are whether it's mined or synthetic (if it really did come from Afghanistan, it's mined), and what sort of treatments it has had.
Very large emeralds are unfortunately often built out of emerald dust and fragments and are not very valuable. There are many of these on e-Bay. On the other hand, it could be worth a great deal.
Your best bet is to post in colored gems and let them help you find an expert.
That’s not a flippant question. I get asked about stones like this fairly often, and usually, it’s couched in the form of a business proposition. Someone in Afghanistan sold it, or still has it, and has described it as a profitable transaction for an American to bring it back to the states and sell it. This rarely to never works out. The problem isn’t usually with either the grading or even the 'value'. Selling a 350 carat low-grade emerald is HARD at any price. There's not a viable market. Jewelers don’t take them into inventory because it’ll take forever for the right customer to come along. Sellers tend to think they’re worth giant money because there are charts online that say they are. There are appraisers who will write a report that says they are. It’s a vicious and circular argument. The best market in the world for this seems to be selling to Americans temporarily stationed in Afghanistan.
If what you're looking for is an estimate of the retail replacement value in the US market, there is a list of appraisers under the resources tab at the top of the page. That's a very different question and will get a very different answer.