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The lockets are the shape of monster souls, they are usually white but when a monster absorbs a human soul, their lockets adsorb the soul and then it starts rotating colors. (It turns white, then the human soul color,etc.)
With the human souls, they are the same as their soul, but with color.
The AU is based on the heart locket.
When the first human fell into the Underground, Alphys had gone through a lot of trouble designing the first locket for the human, which included destroying a few amalgamates just to get the DETERMINATION needed for the human Heart Locket to work, but after a while the child's plan went into action and Asriel had the human's soul in his locket, doing what the human wanted. Then his locket broke, and it all went down.
The human's cracked heart locket can be found in the gifts in Asgore's home.