Some are of limestone, others of silicic matter.

The favourite stones, when the neighbourhood of the nest permits, are little nodules of quartz, smooth and semitransparent.
These are selected with minute care.

The insect weighs them, so to say, measures them with the compass of its mandibles and does not accept them until after recognizing in them the requisite qualities of size and hardness.
A circular fence, we were saying, is begun on the bare rock.

Before the mortar sets, which does not take long, the mason sticks a few stones into the soft mass, as the work advances.

She dabs them half-way into the cement, so as to leave them jutting out to a large extent, without penetrating to the inside, where the wall must remain smooth for the sake of the larva's comfort.