In the center of some silk plants they have a piece of metal in the middle so you can bend and curve the stems the way you like them. You can use a pliers to pull out the metal. But i think the silk plants for aquarium use dont have the metal. I have some and they are just made out of plastic and silk.
We are talking about silk plants from hobby stores that aren't meant for tanks. Since they aren't meant for tanks, the metal they contain isn't either.
These plants are a popular choice because they are much cheaper than aquarium plants.