Calipers would be accurate, a micrometer would probably be most accurate. there's a few options, depends on your purpose.
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What do you want to measure about the marble? Its diameter, radius, circumference, volume, mass, density...?
I would use a micrometer. A caliper would do as well.
14mm
That depends a lot what you want to measure: its diameter, surface area, volume, mass, density, color, hardness, etc.
Internal diameter.