Everyone has different body chemistry, so it's good to be aware of what you have sensitivities to in order to make an informed decision on metal choice. Keeping your jewelry clean is important. Wipe your jewelry piece with a soft cloth before you put it on or after you take it off, and don't shower or sleep in your jewelry.
Niobium is hypoallergenic, so even those who have trouble with other earwires should be able to wear these. That said, the term hypoallergenic simply means "less likely to cause an allergy," it does not guarantee that your ear piercings will not react.
Sterling silver Aside from my own hand crafted sterling earwires, I use primarily Bali silver earwires which are authentic and handmade by artists in Bali, Indonesia using genuine *925 sterling silver. Pure silver is relatively soft and malleable, and easily damaged so not well suited for certain types of jewelry. To defend against deformation or destruction, silver is combined with other metals to make it more durable. Sterling silver is the most common alloy mix found in jewelry. It must be at least 92.5% pure silver, *hence the designation 925, but the other 7.5% can be any metal. Typically, this alloy is copper. Centuries of experimentation have shown copper to be silver’s best companion, without affecting its beautiful color.
Gold filled components are composed of a solid layer of gold (typically constituting at least 5% of the item's total weight) mechanically bonded to a base of either sterling silver or other base metal. I never use gold plated earwires because the plating can wear off quickly and expose the base product. It does not stand up to heat, water or wear over time. Gold filled jewelry has almost 100x more gold than gold plated jewelry. Since the gold is mechanically bonded to the base rather than plated, it can't rub off.
Copper I, along with many customers, love the look of copper - especially patinated (darkened) copper, in certain pieces. It just compliments the design so well, particularly in boho pieces. Niobium is a great alternative if you happen to be sensitive to copper, but Sterling also makes a good substitute.