Soul gems are very useful for Mages and Warriors with enchantments on their weapons, there are different ranks of Souls and Soul Gems, you need the correct soul gem to store the same rank soul or any lower ranked soul for example: A Common Soul Gem cannot hold a Greater or Grand soul but can hold a Lesser or Petty Soul. Nor can any regular Soul Gem hold the souls of NPCs. Soul gems are necessary for the Spell Making Altars you need a filled soul gem to Enchant and make Spells. Soul Gems can also be used to recharge magic weapons, simply click on a filled Soul Gem in your inventory and then select the weapon to recharge.
 Ranks of Soul Gems
The rank of a Soul Gem determines how many enchanting points you can store in the gem.
Petty Soul Gems - The Smallest Soul Gems can only hold Petty Souls nothing greater storing a maximum of 150 enchantment points.
Lesser Soul Gems - Are Small Soul Gems capable of holding Petty and Lesser Souls nothing greater, storing a total of 300 enchantment points.
Common Soul Gems - Average size Soul Gems capable of holding Petty, Lesser and Common Souls nothing greater. They can store a total of 800 enchantment points.
Greater Soul Gems - The second largest Soul Gem available is capable of holding all Souls that are not Grand in level. They can store a total of 1200 enchantment points
Grand Soul Gems - Grand Soul Gems are the largest Soul Gems and can hold any Soul in the game except those of the NPC Races. They can store a total of 1600 enchantment points.
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