An aggregate is just a rock, you might think. Not so, because not all rocks can qualify. Crushed stone can be composed of limestone, granite, or any hard rock that is produced by blasting and then crushing.

When you are making a road or building there are physical, chemical, and mechanical properties you have to consider. For example the particle shape can prevent skids at high speed on roadways. If you choose aggregates with smooth surfaces, it will promote skidding. If you use aggregates that are more angular you increase resistance, but then you might increase tire wear.

Size and sorting of the particles are important. When roads are being made, you need to compact the aggregate to make an even surface. If you have too many different sizes then you might have too much pore space. Fluids could seep through and cause erosion under a roadway. That could cause major troubles for vehicles.