Calipers would be accurate, a micrometer would probably be most accurate. there's a few options, depends on your purpose.
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.
What do you want to measure about the marble? Its diameter, radius, circumference, volume, mass, density...?
milimetersMilliliter
millimetres or mm
That depends what aspect of a marble you want to measure, for example its diameter, its mass, its density, its color, etc.
That depends what you want to measure: its diameter, its mass, its weight, its color, etc.
I would use a micrometer. A caliper would do as well.
14mm
6.5cm
If you have calibrated eyeballs
That depends a lot what you want to measure: its diameter, surface area, volume, mass, density, color, hardness, etc.
13.2 cm, if the given measure is the diameter.13.2 cm, if the given measure is the diameter.13.2 cm, if the given measure is the diameter.13.2 cm, if the given measure is the diameter.
Internal diameter.