Soul Gem (Oblivion)

A Soul Gem is an magic stone capable of storing the "soul"&dagger; (spirit or life-essence) of a creature; the spirit energy trapped in these gems are used by Mages to recharge magical weapons, or to create new enchantments.

&dagger; ''The word "soul" appears in quotes here because the word is used incorrectly in the game. For example, a Zombie or Skeleton is, by definition, a soul-less creature, but it does have an animating life-force.''

Soul gems come in various sizes: petty, lesser, common, greater, grand and black. The bigger the gem, the greater the "soul" that can be stored in it.

For a description of the process used to capture a "soul" in a Soul Gem, see the 'Soul Trap' article.

There are a few other ways to charge Soul Gems aside from using the 'Soul Trap' spell (if you are not of a high enough skill level to cast the spell): You can also obtain Soul Gems that already have "souls" in them by buying or stealing them from a mage/wizard.
 * Use a weapon with a Soul Trap enchantment, such as Umbra.
 * Use a Soul Trap scroll.

Special Soul Gems
'Azura's Star' is a reusable Soul Gem, obtained by completing Azura's task at the Shrine of Azura.

Black Soul Gems hold humanoid souls (Imperial, Argonian, Khajiit, etc, as well as Daedric kyn). All others, including Azura's Star, hold only the souls of creatures: animals, or the non-humanoid daedra found near or inside Gates of Oblivion. They are created by placing a Grand Soul Gem in one of four necromancer altars in Cyrodiil, then casting Soul Trap on the altar at the appropriate time. They can also be found rarely on necromancers. All humanoid souls have a grand value. *Note that humanoid souls have the same value of soul points as grand souls, so going out of your way to make black soul gems after you have access to opponents with grand souls is almost pointless.

Soul gem sizes
1: These creatures can be summoned using Conjuration spells.

A Note on Soultrapping
To Soul Trap without having to cast the actual spell, you can enchant a weapon with Soul Trap (1 second duration). One second is sufficient if the weapon is the only source of damage. If your weapon is also enchanted with a secondary damage source (such as elemental or Damage Health), a Soul Trap duration of 2 seconds (or more, if it is damage over time) is necessary.

Another way if you are a spellcaster is to create a spell that deals damage and add into the spell soul trap 1-2 second duration

You can also soul trap creatures that you have conjured. You must, however, kill the creature to trap the soul. You cannot simply let the spell expire.

The vault guards in Frostcrag Spire in the Wizard's Tower plug-in all have a soul value of Grand and respawn eventually.

Another easy way to fill soul gems is to get a Black Soul Gem and duplicate it with a duplicate items glitch. If you have the Staff of Worms (dropped by Mannimarco during the Confront the King quest), go to one of the Bandit or Marauder camps around Cyrodiil. Kill all of the Bandits/Marauders except for one. Soul Trap this one, and kill it. Then, use the Staff of Worms to reanimate a dead bandit. Wait a few seconds, then Soul Trap it and kill it. It will count as a humanoid soul. Continue this process for as long as you like.