So, no, not a precious stone, but several unrelated similar-looking plants; Wikipedia lists a couple others also called this, but these two are most commonly mentioned in dictionaries. The name is a via a mushing together (in the same manner sailors mushed forecastle into fo'c'sul and boatswain into bo'sun) of French Saint Pierre aka Saint Peter, the patron saint of sailors, so named after because the plants were usually in the sea-spray. As for why you'd want to invoke a saint, I can do no better than cite Edgar's speech in King Lear IV iv:
Come on, sir; here's the place: stand still. How fearful---L.
And dizzy 'tis, to cast one's eyes so low!
The crows and choughs that wing the midway air
Show scarce so gross as beetles: half way down
Hangs one that gathers samphire, dreadful trade!
Methinks he seems no bigger than his head:
The fishermen, that walk upon the beach,
Appear like mice
