If it is pure 10k gold, it will never fade. Think of it like this, 10k gold means that 10/24 of the necklace is 24k pure gold. So even though your necklace is 10k, it is still mixed with other metals likely silver and bronze to add weight. 24k gold is not strong at all and believe it or not, you could bend a solid 24k gold ring with very little effort. Most necklaces that are solid gold 10k, 14k etc… will actually be plated with gold on top of it to bring out the shine in the metal. I would recommend a jewelry cleaner and then finish it off with a jewelers cloth. that should bring a very nice shine back to your necklace.