My background radiation reads about 20cpm on a Geiger, and then reads a piece of jewelry as 40cpm. Is it uranium glass?
To have a true positive reading for uranium content, an above normal amount of radiation must be detected. “Above normal” means higher than regular background radiation. Anything below 50cpm is still considered a "normal" amount of radiation, even if it reads higher than your regular background reading. It's when it hits above 50cpm that it's considered an above normal amount. A similar example would be if you get your cholesterol measured - anything less than 200 is considered normal cholesterol. If yours is 198, it's on the higher end of normal, but it's still not considered high cholesterol.
This does not mean the item is not uranium glass if it reads below 50cpm. Lots of uranium glass does not have enough radioactivity to read above background radiation. If the item is glass, and it glows bright green, then it contains uranium. Even some less vibrant glowing glass like dark blue can still be uranium. A Geiger can be a useful tool to positively identify uranium in pieces with an unusual glow, and as a fun tool in general, but it cannot be used to determine that something is not uranium glass.
Positive Geiger counter reading on a vintage Czech necklace with an unusual, barely green glow. This is a case where having a Geiger is a very useful tool.
Normal radiation reading for a pile of 20 vintage uranium glass chandelier crystals. These crystals glow very bright green so they are most definitely uranium glass, despite no radiation reading.