Type
Marbles were made out of many types of material, including clay, porcelain, agate and glass. The type of material and the decoration on it or in it is a major determining factor in the value of the marble.
Condition
Marbles are susceptible to chipping, cracking and other damage. Any damage reduces the value of the marble, though collectors may disagree on the extent of the damage and how much it affects the value. Some collectors use a system of numbers between one and 10 to designate condition; some use "mint," "near-mint," "good" and "collectible."
Eye Appeal
Eye appeal can be a subjective point of value, but it generally relates to the brightness of the colors and the symmetry of the design.
Size
Marbles were made in many different sizes, and even marbles that were labeled the same size could vary by up to 3/64 of an inch. Some sizes are more common than others, but this varies by material. Less common sizes are valued more highly.
Rarity
Generally, the less common a marble is, the more it is worth. However, if a marble is so rare that collectors have never seen or heard of it, it may not be valued highly.