Sharpham Hall in Devon, by Sir Robert Taylor (1770) offers a good example of decorative treatment of Portland stone. At Sharpham, as at most Georgian country houses, the most important rooms, and hence the ones with the highest ceilings and tallest windows, are on the first floor, so the architect is faced with the problem of how to give the less important ground floor sufficient visual weight. To hold its own in the composition it needs to be made to resemble a plinth or base, so the joints between the blocks of stone on the ground floor are recessed through chamfering, in a process known as rustication. The recesses create shadows, as here, and therefore give an illusion of solidity and strength, even though in actual fact the chamfering means that there is slightly less stone in this 'masculine' part of the facade than there is higher up.
To enhance the effect, the faces of rusticated blocks are sometimes tooled. Quite often they are vermiculated, ie given a surface pattern that resembles worm casts. Or you might see fluting (narrow concave bands) or corduroy work (narrow convex bands). Less common is the effect here, known as punching, where the surface is covered with regular small indents. The intention is partly to create ornament but partly also to reinforce the impression of strength by adding multiple small shadows to those made by the chamfers.
Above the rusticated blocks at Sharpham you can see a projecting moulded string course, which marks the transition between the ground and first floors. And above that, the Portland stone is left smooth, so well cut that the joints between the blocks are barely visible. (Photo: Robert Bargery).