While the Voice of Seren is active in the Ithell district, the soft clay rocks will not deplete. Which means that your inventory will continue to fill with soft clay while mining here.
Even though mining a Clay rock only requires level 1 Mining, entering the city of Prifddinas has a prerequisite requirement of level 75, making the mining site useless for players who have not already completed Plague's End.
The rock mass that surrounds the six individual soft clay rocks does not have an examine text, but individually they all have the examine text: "This rock contains soft clay."
Mining soft clay with a bracelet of clay yields two soft clay per batch. One charge is still used per batch, and the experience is not doubled.