Oysters shells break down in a manner that makes them good for vehicular traffic. The shells, once crushed, become compact and durable. Crushed oyster shells also disperse evenly when rolled over by tires, eliminating the large gaps you see with other rocks and materials. Most oyster shells are off-white or gray.
Clam shells are also a commonly used shell for creating or paving driveways. They are more fragile than oyster shells but still break down well. Whole or crushed clam shells can be used, depending on the desired look. Most clam shells are off-white or yellowish-white.
Scallop shell driveways, though popular in many areas, are less common than oyster and clam shell driveways. They too can be used crushed or whole. Most scallop shells come in white, gray or brown varieties.