Soul Gem (Oblivion)

A Soul Gem is a magic stone capable of storing the "soul" (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.

Information
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 Soul Trap.

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.

Soul Gems are purchasable from most alchemical and magical merchants. Soul gems found empty, whether as loot or bought from merchants, have a coin value which depends on the grade of soul gem, which does not change if the player later fills them with a soul. Merchant gems sold or found filled with a soul have a coin value which depends on the level of the soul within, regardless of the grade of gem.

Special soul gems
'Azura's Star' is a reusable Soul Gem, obtained by completing The Black Star at the Shrine of Azura.

The Black Star (item) is a reusable Black Soul Gem, which is arguably even better than Azura's Star, since it can store the soul of any human character (giving a GRAND soul). It is obtained by completing The Black Star by taking the 'Broken Azura's Star' to the elven mage in Winterhold instead of taking it to 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. Black Soul Gems 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. Note that all humanoid souls have a GRAND value, making the value of a humanoid soul the same as, say, a Gloom Wraith; this equality of soul value makes the creation of Black Soul Gems a basically unnecessary process once you start encountering more grand souled creatures in the game, since the power level of an enchantment from Grand and Black Soul Gems are identical. However, Black Soul Gems do give somewhat of a deeper role-playing aspect for a Necromancer/evil type character.

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

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 used to give the killing blow. If the weapon has a damaging enchantment of more than 1 second, the soul trap effect has to last longer. So, for example, if a sword dealt five seconds of fire damage, the soul trap effect would have to last 6 seconds.

Another way, if you are a spellcaster, is to create a spell that deals damage and add into the spell soul trap with a duration of 1 higher than the damage spell.

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 Greater and respawn in three game days.

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.