Golden Beryl from Maine
There is a long history of fakes and forgeries in minerals, fossils and particularly gemstones. And of course you have all heard of the fake gold nuggets - lumps of lead electroplated with gold.
Fakes are worse than undisclosed repairs and renovations, which are bad enough. I will cover that topic later.
Crystal glued back on the right kind of matrix but it was never on that matrix. common example - Cinnabars from China
Crystal glued on matrix that it never possibly occurred on - like the galena cubes glued into geodes from Morocco
Heat treated and dyed minerals. Though heat treatments are somewhat acceptable in gemstones it is in no way acceptable in mineral specimens. Examples - Heliodors from Tajikistan (though this is debated I believe at least 95% are treated if not all), Tanzanites (most are treated and some additionally have dye injected), some Emeralds from Colombia have dyed oil injected, many blue Topaz are heat treated and the ever famous Brazilian Citrine or as I call it "burnt amethyst"
Lab grown minerals not disclosed as lab grown. Some minerals are grown in labs, note that I say lab grown. They are natural compositions but are lab grown. (there is nothing that is not natural after all, so a unscrupulous dealer could say of course it is natural, yes, but is it lab grown?) Now there is a legitimate market in synthetic minerals as long as it is disclosed. Examples - Green amethyst (sorry, there is no such thing though there are some quartz with green inclusions), bismuth crystals, some rubies, some seed quartz, zincite from Poland (very pretty stuff) and many more.
So buy your minerals from a reputable dealer and ask questions. Even a reputable dealer can get snookered but a reputable dealer will graciously accept a return.
Oh and one more comment. There is a lot of bigotry about these fakes. No particular country has a monopoly on forgers, they come from every corner of the earth.
Page of Website on fake minerals with detailled list of fakes