This question already has an answer here:
The only vivid memory I have of this book is that the citizens of the story's world are all associated with a certain element, such as air or water, etc. This association brings them powers that can control that particular element. The powers, though, can only be enhanced/ used if 'special stones' are implanted in their spine after birth.
The stones themselves are plain, but once implanted in a baby's back, they turn a certain color. The stones are implanted in a special ritual performed by alien-like creatures, who are very powerful.
I have a feeling it was a part of a series, but I am unsure.
I just remembered something else. The stones are very expensive and the parents have to save up in order to afford them. Multiple stones can be implanted in one person and the number of stones implanted in a person indicates the amount of power they have.