Blue Metal is a quarried, crushed aggregate rock that can range in color from blue to black. It comes in three sizes: 5 mm, 20 mm or 40 mm, and is primarily used in making concrete. Because it is a clean, crushed rock, it is also ideal for drainage, particularly around agricultural pipes. Blue metal is a popular choice for constructing driveways or paths. 10mm and 20 mm sized rocks are typically used when making concrete. The larger 20 mm size rocks work best to bind with cement or asphalt and form a better, stronger concrete. Generally, the 5mm size is not recommended for such tasks, as it is considered too small to provide a desired stable base. Please note that colors can vary greatly from load to load and this is a high turnover product, so what you see one day will be different the next. If color is especially important to you, you may wish to inspect the material prior to delivery and on the day of delivery. We recommend getting the whole quantity at one time, for consistency. In short: Be sure to calculate and order all the materials required for your blue gravel driveway or other project in one lot, to avoid the variations in size and color that can occur.