Russian Shungite 
Hematite
Tourmaline
Bare copper wire 
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.