I found another use for gemstones (could possibly be considered a spoiler):

The Atronach Forge in The Midden under Winterhold College can be used to conjure atronachs. You need to put "recipe" items in the offering box and then pull the lever. One of the recipe items is a gemstone.

The "Atronach Manual" lists the following recipes:

Flame Atronach: Fire Salts + Ruby
Frost Atronach: Frost Salts + Sapphire
Storm Atronach: Void Salts + Amethyst

I found another use for gemstones (could possibly be considered a spoiler):

The Atronach Forge in The Midden under Winterhold College can be used to conjure atronachs. You need to put "recipe" items in the offering box and then pull the lever. One of the recipe items is a gemstone.

The "Atronach Manual" lists the following recipes:

Flame Atronach: Fire Salts + Ruby
Frost Atronach: Frost Salts + Sapphire
Storm Atronach: Void Salts + Amethyst

Also, gemstones + soul gem + salt pile can make either frost salt, fire salt or void salts, depending on the gem.

I found another use for gemstones (could possibly be considered a spoiler):

The Atronach Forge in The Midden under Winterhold College can be used to conjure atronachs. You need to put "recipe" items in the offering box and then pull the lever. One of the recipe items is a gemstone.

The "Artonach Manual" lists the following recipes:

Flame Atronach: Fire Salts + Ruby
Frost Atronach: Frost Salts + Sapphire
Storm Atronach: Void Salts + Amethyst

I found another use for gemstones (could possibly be considered a spoiler):

The Atronach Forge in The Midden under Winterhold College can be used to conjure atronachs. You need to put "recipe" items in the offering box and then pull the lever. One of the recipe items is a gemstone.

The "Atronach Manual" lists the following recipes:

Flame Atronach: Fire Salts + Ruby
Frost Atronach: Frost Salts + Sapphire
Storm Atronach: Void Salts + Amethyst