Most amber sold badly is not plastic. It is copal — tree resin that began the same process and never finished it. It looks convincing in a photograph and behaves quite differently in the hand.
State, not age
The usual way to separate the two is by date: amber millions of years old, copal only thousands. We do not use that argument, and it was never the useful one anyway. What actually distinguishes them is how completely the resin lost its volatile compounds — and that depends on heat, pressure and how well it was sealed, not on a number you cannot verify.
What gives copal away
The solvent test is the one dealers rely on, and it also destroys any finish it touches — which is why nobody should do it to a piece they intend to keep.
Why it matters at the price
Copal is a legitimate material honestly sold. The problem is a copal bracelet priced as amber, bought as an heirloom, and clouded within a decade. Ask the seller directly. Anyone who works the material will answer without hesitating.