a) It's not an artifact. It is the egg of a creature of such power that it is easily mistaken for a artifact. Fed to much (or enough depending on your perspective) magical energy/life force/wealth and it hatches. What will arrise from it?
b) It's not an artifact. It's is someone's soul. A legendary hero was slain eons ago by a legendary villian. Before the battle, he took the percaution of locking his soul away in a priceless gem. During the battle he was slain, but he also slew his enemy. If a body is provided for the stone, and the proper ritual performed, a great good will be released back into the world. Unfortunately, so will a great evil...
c) It's not an artifact, it is prison of a fallen angel.
d) It is an Artifact. It is the Propylaeum Revephaktory. This artifact unlocks the way between the real world, and the world of possibilities, dreams, and illusion. It is rumored to have some or all of the following powers, and these are probably but a tiny portion of its powers in the hands of a wizard who has studied its potential: Ghost Sound (at will), Augury (at will), Contact Other Plane (once per hour, but always contacts equivalent of astral planar or higher level being), Disintegrate (once per day), Gate (once per day, called creatures always have the Paranormal template (not the ELH one!)), Wish (once per week), Maze (three times per day), Minor Creation (at will), Mirage Arcana (once per day), Nightmare (once per day), Phantasmal Killer (three times per day), and Shades (three times per day). But mortals best beware the awesome power of the stone, for its keepers seldom survive long enough to impart its secrets to others. They have a chilling habit of simply disappearing never to be seen again. (Object turns its powers randomly against whoever remains in proximity to it, as well as other more esoteric complications).
e) It is an Artifact. It is the Orb of the Everwell. This artifact is sacred to the Marids, and is said to contain the pure primal essence of water locked safely beneath its imprenetrable surfuse. Should an evil being find a way to release it, he might also find a way to control and shape this essence to better suit his wishes - resulting in water throughout the multiverse becoming a tainted thing of evil rather than the blessing it normally is. The bearer of the stone has the fulls powers of a Ring of Elemental Command, and gains other powers as it is mastered. But there are likely to be complications for carrying around something with greater influence over the nature of the cosmos than most deities.
f) It is an Artifact. It is the Heart of the World. This artifact is (in modern terms) the central processing unit governing the motion of an entire world (either the earth or its moon or some other planet conveinent to the setting). It has the following known powers, and likely many other powers besides: Triple Widened Earthquake (at will, but always centered on and effecting bearer unless a spellcraft DC 80 check is made), Reverse Gravity (at will, but always centered on and effecting bearer unless a spellcraft DC 90 check is made), Control Winds (at will, as above, DC 60), Control Weather (at will, as above, DC 70), Control Water (at will, as above, DC 40). Don't drop it...