Absolutely no way to tell from your photograph.
Does it look like amber to you? I mean, do you see natural irregularities, tiny bubbles, inclusions of any sort that you expect to see in tree sap?
Does it smell like amber to you? I mean, when you rub it until it's warm, do you get faint smell of natural resin, of tree sap, sort of a pine-y smell?
Does it feel like amber to you? I mean, is it warm? (Yes, I know that some plastics feel warm, too, at least in comparison to glass.)
Does it take a charge? I mean, if you rub it vigrously on wool does it build up static electricity to the point that it will pick up a strand of your hair, say, or a tiny scrap of paper? (Yes, some plastics will do this, too.)
I'm not going to go into the whole specific gravity thing because I dont think you want to put this in water.
Anyhow, is there something besides the color and the glow under blacklight that makes hyou think this is amber?
