Rather than a heat-treatment (which could be destructive) try
this: fix yourself a nice cup of tea, take the used leaves and
add just enough warm water to cover the amber, leave out in the
sun a day or two and check it often…if you need a darker color
yet try a darker tea leaf.
Some amber is more porous than others and if it isn’t coated or
too highly polished I think you will be surprised and happy with
No guarantee, but it does work sometimes…