Items:
Life Spirit Stone (ID: 45302)
Guardian Spirit Stone (ID: 45300)
(Destruction Spirit Stone (ID: 45298) not tested, but most likely same behaviour)

Short version:
Buff granted by Spirit Stones is invisible and there is no durability consumption.

Long version:
Should work like this: Equip stone -> press hotkey -> player gets buff -> durability of Stone gets decreased.

But actual behaviour tested with Guardian Stone: Equip stone -> press hotkey -> player gets 110 HP -> press hotkey again -> message pops up: "Skill unavailable. (Group Cooldown)"
Seems like to buff gets applied but there is no buff icon show and the durability of the Stone is not reduced.

Tested it again with the Life Stone. Now Gathering gets 2 additional levels (so the buff gets applied) but no icon / durability consumption