The most used is lead.
Lead
Lead
Lead is most commonly used for this. An example is the lead jacket the put over you when you get an x-ray at the dentists'.
Lead is commonly used as a shield to protect people from radioactive substances. It is dense, so it effectively absorbs and blocks the radiation, helping to minimize exposure to harmful radiation._lead
Bella is a shield she can protect people
edward as you know can not read Bella mind when Bella becomes a vampire she discovers that keeping people out of her mind is her power. AKA a 'shield'. she learns to push it away from her to protect other people.
Usually lead is used for shielding, however I believe Boron is used to in a sense "neutralize" radioactive material. I believe this was discussed in possible solutions for Fukashima
A Shield to Protect a Knight from injurys
they step in and become like a shield to "defend" and protect the people, weapons etc...
The knight use he's shield to protect his self.
Bella is a shield and can guard (protect) herself and other people as well
someone may use a shield to protect or cover something