Nickel allergy is one of the most common causes of allergic contact dermatitis.
Earth friendly alternatives are Copper, platinum, and sterling silver.
Nickel isn't the only metal that can cause allergic reactions, although others are much rarer. Doctors are increasingly finding allergies to palladium, a platinum-like metal that is gaining popularity. Palladium is often used in white gold alloys.
Platinum itself is generally considered safe from allergies, as is 24-karat gold, copper, sterling silver and stainless steel. Dec 15, 2009
Mayo Clinic: “Nickel Allergy” http://www.mayoclinic.org/diseases-conditions/nickel-allergy/basics/definition/con-20027616
Wall Street Journal: “What To Do When You're Allergic to Your Wedding Ring” http://www.wsj.com/articles/SB10001424052748703954904574595943916863808