If the soil is so hard it cannot be dug, roots will likewise not grow in the hard soil. If there is a combination of two or more it is called a loam. A clay loam is mostly clay, but it also has sand and silt with it. Loams are always better. The sandy soils drain very fast, but have a very low water holding capacity.
Clays have high water and nutrient holding capacity, but drain very slowly.
Gravel or small rocks in a soil can help internal drainage which is good. From time to time growers will here someone talk about soil structure. It is easy to use the word and it is sometimes used with some understanding of what it means, but most of the time growers and scientists do not know much about soil structure, other than know it is their problem. Good soil structure can be called soft soil, while poor or better yet, the absence of structure is hard soil. Good soil structure comes from good soil chemistry where clay particles are pushed apart by ionic charges.