All real Baltic amber comes out of the ground in only a small range of light colors (other than the green ones which are pretty dark to begin with). In order to get all of the other colors of amber (cognac, dark cognac, dark cherry, etc.) they must be slightly heated or pressurized (with an autoclave) or both.  We learned more about this process when we went to Lithuania to visit the workshop where our amber is made. You can see pictures of the machines that they use to do this process on this page.