What is the difference between black and grand soul gems, aside from appearance?....I remember there was a quest in oblivion about them...and you could only get them by placing grand soul gems in alters in remote locations at midnight on a full moon or something......is there any benefits to them over grand soul gems though?
The main difference is that Black Soul Gems can capture humanoid souls, but only humanoid souls. Other soul gems (Petty, Common, Greater, Grand) only capture creature souls. This has an advantage as all humanoid souls are considered Grand, where as animal/creature souls scale based on level (weak creatures like a skeever give a petty soul and big creatures like a mammoth give a Grand soul). As for the fragments they may just be miscellaneous loot (found items such as Broken Steel Sword Hilt as well) or they may be implemented later in some DLC as quest items. For now I have a pile sitting in a drawer in proudspire manor, they seem to be fairly common in dungeons with spellcasters (usually find them littering tables or near an enchanting table).
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