"Calipers" I get mine from Harbor freight, I found the plastic ones easy on glass and accurate enough for marbles (& they're cheap). Make sure you get the ones that measure in "fractions of an inch" Most sets measure in inches, fractions of an inch and mm. Here is a link for ya !!
LINK :: https://www.harborfreight.com/6-in-utility-caliper-63664.html
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The top is inches and fractions of an inch & the bottom is mm. These will measure inside and out side diameter, outside is for marbles and such and inside is for measuring the diameter (width) of a hole. W/ marbles all you'll be dealing w/ is the top of the caliper ( inches & fractions of an inch) not many use mm, mostly out of the continental US. The mib pictured is 27/32nds of an inch, don't worry, you'll catch on fast