Some parts of what Judy said is true. Other parts are unfortunately
not so true. One terribly glaring error is as she stated, amber from
the Dominican Republic is not amber. In fact it absolutely is.
Baltic amber does tend to be older. But some Dominican is quite old
and all is in the amber category. What is called amber from Columbia
is almost certainly copal.
As to amber treatment. Untreated is no more brittle than treated. If
anything it might be the other way around. I doubt spangles relieve
any stress but I do know they are used primarily for decoration. I
can't say for sure about their brittleness. I will say that the
longer amber is exposed to the air and can oxidize the more it is
likely to be brittle.
In general amber will break easily but that's because it's very
soft. Mohs 2-3.
The Polish green amber is most assuredly treated and with a green
backing. There is however, blue and green amber from both the
Dominican Republic and southern Mexico, the province of Chiapas.
Neither of these craze after cutting any more than Baltic. But after
many years, say 50 to 100 they will tend to get more brittle.
There are various shades of blues and greens and some are only a
hint. On the other hand some blue amber in the right lighting.
specifically full spectrum is very clearly blue. It shows up best in
daylight. The natural greens tend to be more tints than colors.
Well that about covers much of what I know. But I have looked into
this stuff a bit.