Hi Maria, yes it is. Usually in nucleated pearls, this effect is minimal to not being there because the nacre is so thin. In a larger natural pearl like the ones from Feb and March 2008 in Toms Stern's natural pearl thread, it is more obvious and can be sort of horny looking. You can see the red through a lot of the pearl, but it is less desirable in a baroque because of the horny look.
We don't have an example of water in a round natural pearl on this forum, but it would be far deeper than the cultured one. And it immediately transports an otherwise perfect pearl to the top of desirability.
Water is translucent, but it also reveals its structure, because in some lights, the iridescence sheets to the edge. Really thin sheets.This means you see the sheets and not the translucence. This sheeting affect is nothing like I have seen before. Its like a diamond crown flashing around the outer edge that alternates with the translucence. I spent more time with that pearl, than I did any of Tom's other beauties, not because it was beautiful, but because it was so fascinating.