Anodized aluminum, my most common material, has a high quality, non-toxic coating on it that is FDA approved for use in cook ware and will not stain or irritate skin.
Bright aluminum, however, does not have the same coating and can cause some dark staining on skin, though this will wash off in soap and water and is non-toxic. This material is therefore only used in costume pieces, not jewelry, and can always be swapped out for any other metal that you prefer.
Most other metals are non-allergenic, such as titanium, stainless steel, brass and copper.
My earhooks are either stainless steel or silver-plated, and earrings can be made either way according to your preference.