The Hindu (or whichever system you like) idea of the chakras links colors to energy vortices that supposedly exist in the body. Choosing stones on this idea is colorful and fun.
Amethyst is said to help with addictions, and other physical, psychological, emotional and mental issues that trouble people are said to be helped by using certain crystals and gemstones. Choosing stones based on this premise may be healing or otherwise helpful, even if only used as an intention-focusing device. Some say that what we believe and focus on, we bring into manifestation.
Using crystals and gemstones to focus your spiritual attention and energy to transforming and spiritual endeavors may be enlightening.
Do you think that a certain planet needs to be symbolized by a certain gemstone, either because you just think that or because you heard or read that it was? Whatever you want!
Do you associate certain stone with Magickal properties? Want to cast a love or abundance spell on yourself...Use those stone based on their so called magickal properties! Just tell me what you want - I will make it for you...
Do you believe that a certain stone has a vibrational frequency that is particularly suited to some thing you want? How about, we put that stone in your chart piece? In fact, if you are so attracted to a certain stone, let's put a lot of it in something for you... whatever. you. want.
You will be intuitively drawn to certain stones. I suggest going with those feelings. You also may find you want to wear different colors based on the colors you wear in varying seasons. Perhaps a pastel piece, for springtime, and darker colors for winter, brighter colors for summer, or more earthy autumn colors for your fall wardrobe.
It's all up to you, what you prefer. There are no rules, you can make each sign or planet any stone you like. I do have some suggestions on choosing them, but they are suggestions only for those who might get a little bogged down with all these decisions, or need some inspiration.