The pebble in the brook secretly thinks itself a precious stone

Here and there a tawny brook prattled out from among the underwood and lost itself again in the ferns and brambles upon the further side. Save the dull piping of insects and the sough of the leaves, there was silence everywhere--the sweet restful silence of nature.

Nor sword nor spear the stripling took, / But chose a pebble from the brook.

You can't get blood out of a stone

It takes a rough stone to sharpen the edge

Even a thief takes ten years to learn his trade.