Yes, it's part of the main plot - you have to solve the alchemist quest - though I don't quite understand your sentence.
Are you locked up under the stone? And can't ascend anymore? if so => part of the plot.
There is just 1 stone. I did as the other guy before mentioned pick up an ingredient before I talked to the Alchimist. So I am standing at the place where the map indicated 2 stones next to each other but in 'real' I see only 1 stone.
