Shungite 
Hematite
Black Tourmaline
Bare copper 
Antique copper bells: copper tone finish over a copper base alloy
Shungite is known to harmonize EMFs, or  man made electromagnetic fields, that are the result of electromagnetic radiation. Wearing this stone is said to protect one from free radial output.
Black tourmaline contains a high concentration of iron, which is specifically good at absorbing and harmonizing EMFs. The magnetic nature of iron literally pulls the electromagnetic frequencies towards itself.
Hematite protects against electromagnetic smog.