This was indefinitely inspired by the fact that Stefan and I have watched all of season 2 and 3 in the last few weeks.
The tree is a tree of life, and the gemstones represent the 4 elements. Light blue = air, dark blue = water, green = earth, red = fire.
Any avatar fan knows that these are also the four nations on the show. But at the most basic level the necklace is a reminder about the earth, and keeping the delicate balance of nature. (That happens to also be the theme of the show.)
Off to go hang out with the amazing AUTUMN!