I would use a micrometer. A caliper would do as well.
Calipers would be accurate, a micrometer would probably be most accurate. there's a few options, depends on your purpose.
Grams
If I was you i would use millimetres or centimetres.
You should clarify what you want to measure about it: its diameter, mass, weight, color, etc. If you want to measure its diameter, that would usually be specified in microns (= micrometers).
milimetersMilliliter
I would use a micrometer. A caliper would do as well.
That depends what aspect of a marble you want to measure, for example its diameter, its mass, its density, its color, etc.
Calipers would be accurate, a micrometer would probably be most accurate. there's a few options, depends on your purpose.
6.5cm
The instrument used to measure the diameter of spherical objects is called calipers.
A vernier caliper
Grams
If I was you i would use millimetres or centimetres.
You should clarify what you want to measure about it: its diameter, mass, weight, color, etc. If you want to measure its diameter, that would usually be specified in microns (= micrometers).
The ceilometer instrument.
A tape measure.