Thank you for your answer, arrived from the sky when already I was adjusting to the idea that no answers would come.
And a heart-warming answer too!(By the way, I hope you are feeling better).
What you say matches my own (unauthorized) opinion.
I purchased the seal as Sassanide, but after some research I noticed that: a)this kind of off-white, opaque agate was not common in Sassanide times b)the actual carving technique does not look Sassanide to me, c) last but not least, the carved symbols are Mesopotamian!
I agree with you: the Moon, the Sun, the Seven Pleiades and a comet (?) can be seen.Clearly a seal of religious or amuletic nature.
No need to say I feel elated.