For the first few thousand years of ear piercings, people didn't give much thought to what metals or materials their earrings were made from. Indigenous people worked with what they had, and not too many cultures were practicing ear piercing.
Jump forward to the late 20th century and early 21st century and ear piercing is now one of the most widespread body art practices you can find on the planet. Choosing what metal or material you wear in your ear piercings now is something that requires thinking about allergies, potential healing problems and if the material is legally-sourced or comes from a sustainable resource. Here is a basic guide to various material used to make earrings.
Acrylic - This is a clear or colored plastic. Benefits are that for most people it is cheap and very lightweight. Drawbacks are that it is porous and can cause reactions while healing. Clear acrylic can also discolor over time, becoming yellowish or cloudy.
Bone - Most often whitish in color and with smooth, shiny surface, bone is something that is non-reactive earrings but only works for larger gauges as it is carved and can't be made into something very thin without the risk of breaking. This material can be questionable depending on what type of bone it is and how it is sourced.
Brass - This is one of the "bad guys" of the piercing world. This is a low-quality metal that is often mixed in with better ones, usually to fake the appearance of gold as it is also yellowish. If you earring starts to get bright green and crusty, it's brass and you need to get rid of it ASAP as it can cause healing problem and sensitivity reactions.
Glass - Glass is made primarily from melted sand (silica) and can be a nice, non-porous earring material. Due to a necessary thickness for strength and to prevent breaking, this is best for large gauge piercings. Glass earrings can be clear or nearly any color.
Gold - One of the highest-quality metals available for body jewelry and also one of the most expensive. For some people with skin sensitivities, this is the only metal that doesn't bother them. Yellow in color, gold can be made into almost thickness, shape or style of earring.
Horn - Earrings made from horn are most often carved from water buffalo horns sourced in Asia. These can be sustainable, as the buffalo shed their horns and grow new ones each year. The colors can be black, brown, yellow-gold, white or a mixture. This material is also most often used for larger gauges and tends to be shaped and carved by hand.
Nickel - This whitish metal is much like brass in that it is a low-quality metal that is often mixed in with high-grade materials to lower the cost. Many people are sensitive to this metal and it should be avoided. If you have silver-looking jewelry that tarnishes greenish, there's nickel in it and you need to get rid of it and get new earrings.
Silver - Pure silver is called "sterling silver" and while it's a nice, bright white metal, it does come with a few warnings. It can NOT be worn in new or still-healing piercings, as it tarnishes (blackens) and this can permanently stain the healing skin.
Stainless Steel - This metal is strong, lightweight and gives the same appearance as silver while actually being made of something much stronger and less reactive. If you have very sensitive skin, you will want to look for "surgical stainless steel" as it is a higher purity than non-surgical grade. Poor quality stainless steel (often from China) can have nickel in it.
Wood - There are many different types of wood that are appropriate for making earrings, and like other natural materials such as bone or horn, these work best for the crafting of large gauge earrings. Hardwoods are more appropriate than soft ones and as with other naturally-sourced substances, you will want to check to see if the source is sustainable or not.