A stereo magnifier may be overkill, but I paid $35 for my used one so it was a steal... only wish I could take photos through it. It is great for detecting fakes (die cracks., renaming etc.).
As mentioned, my mobile magnifier (10X) is one of those 'flip out' jeweler's magnifiers (with light and UV light). Small enough to fit in any pocket.
I put a lot of time into researching the market before choosing it and could conclude "don't buy too cheap". Some of the cheaper magnifiers with a big lens are useless because you have to center the eye within the small middle area to get a distortion-free image (and get cross-eyed in the process...).
Ironically these cheap types are actually sold in optometrist shops to people who really need quality magnifiers in their daily life for reading etc...