Final answer:
Graphite is the crystalline carbon allotrope measured on a scale from 1 to 4 for hardness, with '2' being the most common, due to its relatively soft nature compared to diamond.
Step-by-step explanation:
The crystalline carbon allotrope that is measured on a hardness scale in the U.S. with '2' being the most common is graphite. Diamond and graphite are both allotropes of carbon that have different physical and chemical properties due to their different atomic arrangements, despite carbon atoms forming four bonds in both cases.
Diamond is known for its extreme hardness and can be rated 10 on the Mohs scale of mineral hardness, not on the scale mentioned in the question.
Graphite, on the other hand, is much softer and is commonly used in pencil lead. Fullerenes and amorphous carbon are other forms of carbon but are not commonly measured on this type of hardness scale.