Amber and Heat

I was at the a gem show this weekend and came across a booth that had
some lemon yellow amber with black designs. When I asked the gent how
this was done, he said heat but wasn’t able to tell me more due to
customers. Has anyone heard of this process and how would it be done
? He had some lovely designs on the amber and I am thinking I would
like to try this out.

Many thanks !

Tangible Dreams