It's fairly common for people to be allergic to metals, and nickel is a frequent culprit. Nickel is often added to inexpensive jewelry, which can cause issues. Sticking to pure metals such as gold and silver generally helps to avoid allergic reactions, but some people can still be allergic to these metals, so they're not considered completely hypoallergenic. Titanium is a good choice for people with sensitive skin who are allergic to most other metals.