How long dose it take for tree sap to turn to amber and is their any way to speed up the process?

well i want to make like an amber necklace for my gf. but all the ones i can buy are much too expensive all ways been told that hand crafted gifts come from the heart so i thought id make one my self. so any one know how long it takes for tree sap to turn to amber and is their any way to speed up the process?