French whisk handle has rocks in it?
It could be sand, used to weight the handle. The weight is an important part of the design. I assume some manufacturers use sand instead of just thicker/heavier steel because it's cheaper and easier to manufacture.

For proof that at least some whisks are made like this, here's a whisk with Amazon reviews that mention sand and even a picture:



It's just inert weight. It's not going to come into contact with your food. I guess if you get a defective one and the handle comes open and dumps sand in your food, you'd have to throw it out, but otherwise it's not really anything to worry about.