Shortly before I could finish the underwater part of the pillar, this part of the stonework slided off from the table and...  broke into pieces.
So, back to square one and here ist the result! I tryed to create those stones more porous as they had been exposed to the water.

The colors seem to me still too vibrant and I have to tone them down a bit. But the greenish shine on some oft he stones I discovered more by accident than by experiments. I added some grass flocks to the cast compound and those green flocks gathered at the deepest point of the mould, giving the stones a slight mossy appearance.