Answer:
You speak this when you are being sincere.
One might hang a locket from this
You can have it, and be at it, But it never lasts forever.
A precious gift, yet it has no end, no beginning, and nothing in the middle.
You roll it or you buy it, People say you shouldn't try it, Because you may get a stroke, From inhaling all that smoke.
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.