What is the jelly made of in jellied eels?

Jellied eels are made with eel flesh that has been cooked and then set in a gelatinous stock.

The gelatin is typically derived from eel bones and skin, which are cooked and strained to create a rich, flavorful broth. This broth contains collagen, which transforms into gelatin when cooled, creating the jelly-like texture.

So, the jelly itself is mostly gelatin, derived from the eels themselves.