The turian homeworld, Palaven, has a metal-poor core, generating a weak magnetic field and allowing more solar radiation into the atmosphere. To deal with this, most forms of life on Palaven evolved some form of metallic "exoskeleton" to protect themselves. Their reflective plate-like skin makes turians less susceptible to long-term, low-level radiation exposure, but they do not possess any sort of "natural armor". A turian's thick skin does not stop projectiles and directed energy bolts.
Although life on Palaven is carbon-based and oxygen-breathing, it is built on dextro-amino acids. This places the turians in a distinct minority on the galactic stage; the quarians are the only other known sapient dextro-protein race. The food of humans, asari, or salarians (who evolved in levo-amino acid-based biospheres), will at best pass through turian systems without providing any nutrition. At worst, it will trigger an allergic reaction that can be fatal if not immediately treated.