Tomatoes may well be available in Tibet, and they certainly have (Yak's) cheese, but I question your pineapple suggestion. I believe it's quite the wrong climate.

I must confess that I know less about amber than I do pineapples. I don't know nearly enough about Amber to have any opinion about what those stones really are. Indeed, I didn't know that bright yellow amber was rare or even particularly valuable (I should have guessed cos bright yellow is most beautiful). Duly noted, of course.

I can point you in the direction of a few things to look at. On the one hand, please refer to the posts by Vikuk down the bottom of this 2007 BC.N thread. He visited the second biggest horse festival in Kham, in Litang (currently banned because a nomad called Runggye Adak made this inpromptu speech.
Amounst other things, Vikuk said "Here are a couple pics of modern amberite type jewelry being 'used' in Tibet (Sichuan Tibet).
The amber is some type of plastic/amber powder mix - and some of it actually gives an amber smell when you rub it. Some of it is pure plastic."

On the other hand, I have seen some sellers of Amber, such as this guy who have sold amber in Tibet. Also I have exchanged emails with a former poster on these boards (let's call him 'Mr.Bead') who I understand knows more than most people about beads, and amber especially. He said that he thinks the amber shown in the photos in this thread are baltic amber. As well as the appearance in those photos, that was also based on many anecdotes: he had knows/knew amber dealers in American whose biggest customers had been Tibetans, as well as meeting somebody who had brought back a strand of 5 inch diameter amber beads from Tibet.

I hope some of that information is useful. My suspicion is that many grades of amber, plastic at the bottom, powdered mixes in the middle, and real baltic amber, are all available and worn in Kham, including at these festivals.

I was reading Jewellery of Tibet and the Himalayas by John Clarke (which I highly recommend, if you haven't read it), I think I nearly but not quite got to the bit on Amber... He has photos from the Yushu horse festival in the book, including these amber headdresses. The captions to those images just said "huge amber beads", and did not say they are fake. I will endeavor to finish read the chapters in the book.

Again, if you haven't, please take a look at the high-res photos in this thread, where there's a much closer view of the amber/cheese/lemons. I will try and post more high-res photos at a later point. Now I'm going to bed.