When buying jewelry or other items made of silver, look for the “.925” stamp. This is an alleged measure of the silver content of the piece and is the standard.
This said, be cautious of buying silver from wandering salesman, even if a stamp is visible. This is because much of the silver-colored jewelry sold by wandering vendors is actually made from alpaca, a lesser quality metal with a heavy copper content.
When buying silver or gold an established store is your best bet.
Believe it or not but you can make fake amber. This is not to say that the practice of selling counterfeit amber in Mexico is the norm, but it can happen.
You can tell fake amber because if you put a flame under real amber it will not burn or melt as plastic fake stuff does. If a wandering vendor shows you a piece of amber with a scorpion in it then it is 100% fake because these pieces of amber are EXTREEMLY rare and some guy wandering the streets or beaches is not going to be carrying a piece of amber like that. Best to buy amber from a store unless you can tell the difference.
Also a vendor will say almost anything to make a sale. If the item in question is green and you ask if it is green jade then they will most likely say yes it is, even though it is green marble.
Some vendors also try selling fake Black Coral. Polished black jade can even pass for Black Coral if you do not know the difference. Black Coral is an endangered species and it is illegal to bring it into the US or Canada.
If you do not want to get ripped off, you more or less have to know what you are doing when it comes to buying silver/gold or precious stones.