just making some glass
C & C are welcomed
The caustics are nothing short of incredible, no “photon-spots” or
anything - great!
The object is suffering from lack of antialiasing in the refractional
highlights (jagged - visible - edges, often inside the raytraced object).
These are especially visible inside the black darkened areas.
What J.O. says is probably due to too low Ray bounces, or depth. You might have it low. when multiple internal reflections goes beyond that limit result is black…