I've bought enough gemstones online to finally receive a fake!
The item in question was a masterfully cut, swiss blue topaz, 26 carats and IF clear. I had a winning bid of $41.
So when it arrived, the first thing I did was throw it up on my Refractometer . . . and it registered as GLASS. Whoa. I used my Dichroscope, not true to a blue topaz. Then I did a scratch test. I grabbed a faceted moonstone and sure enough, it scratched the side. Moonstone is a 6-6.5 hardness and topaz is an 8. GLASS, is a 5.5-6.5, depending on the type.
I contacted the seller and he insisted it's not glass but agreed to give me a refund. But it did get me thinking . . .
if you search on Ebay, you can find LOTS of lab-created ROUGH. BIG BLOCKS of it. Think of Moissonitte. There are various types of quality and the same is for other stones. Some Lab stones are better than others. I bought a parcel of 15, 6mm rubies on e bay for $12. STEAL of a deal. They claimed to be real but I was fairly certain they were fake. But for 412, why not?
When they arrived, I tested them. They looked nothing like the photo, all stones were identical incolor and clarity (IF). usually a red flag for rubies. I instantly figured they were glass, but I tested them with my lab equipment anyway.
And they tested as . . . .
RUBY. Everything tested correctly. Harness, refractometer and even the Dichroscope were correct for real ruby. The ONLY way I could not test was with selective gravity (this is an easy test anyone can do with only a tiny scale). They were too small to attempt this. Every stone has a different density, and weighing them in water measures the density, and you can look up the values online.
So I am LEANING toward GOOD lab rubies, although there is a small meek chance they could be real, but for less than a dollar per stone, I'm still happy. I've seen lab rubies this size sell for $100 each.
Lab stones are nearly identical to mined stones, and labs can even replicate flaws and inclusions now, so emeralds and sapphires and rubies are all at risk. And the ONLY way to know is by mineralisys in a lab.
Commenters online have said that large TV sellers tend to be at highest risk for fakes since they require large amounts on inventory of a specific quality. And as I mentioned, NORMAL certification will NOT catch this. Just because a stone is 'certified" does not mean it is what it claims to be

Although I did just snatch a very nice pigeon blood 1.67ct ruby with nice clarity for $120 at auction