Pure gold is 24kt, but is soft. The lower the karat, the less expensive it should be, and the harder and more durable it is, but also, yes, it tarnishes (oxidizes) more.

18kt = 75% Gold, 25% alloy metals, and tarnishes very little.

14kt = 58% gold, 42% alloy metals, and does tarnish over time, and is called the patina finish.

10kt = 42% gold, 58% other metals, and definitely tarnishes over time, and needs cleaning to stay bright.

http://www.goldminerjewelers.com/goldand...

smog fumes and perspiration affect it.

Gold shouldn't tarnish. The higher the k the better. Although the higher k will wear quicker as it is softer. 18k is probably best.

I'm planning on picking up a gold initial necklace for my cousin. She's 18 in high school, she takes college classes, and works so I need this necklace to stay strong and GOLD! I don't want it to tarnish at all. I know 14k is better than 10k as far as value, but I heard 10k is better when it came to durability. Will 10k tarnish if worn everyday?