The density of glass varies between 2.2 and 5.9 kilograms per cubic meter, though values closer to 2.5 are more usual. Common glass has a density between 2.4 and 2.8 and is inexpensive enough for glass marbles.
For simple marble manufacturing, softer glasses are preferred as they are easier to heat and work. Harder glasses are more difficult to work but break less easily, a property that is desirable for toys meant for young children. Pyrex glass, a common and inexpensive hard variety of glass, has a density of 2.2, which is particularly low but not far from common glass.