Since your titling this roack practise, how did this form? Did nature create globs of clay and then press them together? Real rock formations occur for a variety of reasons, the most common being:
water - when completely submerged, rocks are worn to *appear* smooth, without edges, although this actually makes them rough textured, as the water removes the finer grains first. A rock only partiallysubmerged will only be worn where the water passes, leading to interesting cuts and bevels.
deposits - some rocks are formed by sediments beign depositied by water. This is typically seen in the form of sand or bedrock, but stalactite and stalagmite formations in caves can be formed through a dripping process.
wind - like water, only less powerful and in the field.
splitting - under imense pressure, rocks can break off, often yeilding sharp, almost polished surfaces.
grinding - rocks may grind against each other, roughing themselves up quite a bit, and often splitting.
pressing - when forces bear down on rocks, they can be pressed very tightly together, forming rather impressive joins. These usually are only exposed when the stone above is worn or split away.
fusing - seen below the earth, molten stone fuses with the materials around it. This usually can be seen on cliff walls which have been worn away to reveal ancient volcanic activity in the region's bedrock.
Thinking about the forces involved will always lead to a finer product.
