In the Sassanian period the art of cutting seal-stones flourished, as did all the other arts. The shape most frequently used was still the three-quarter hemisphere with flat base created in Parthian times. The distinctive form with flattened sides, oval base and large perforation, however, may have been a creation of the Sassanian seal-cutters, as well as the facets and curving lines precisely and beautifully carved into the surface of the seal-stones. All hemispherical seals are perforated to receive the metal loop by which they were attached as a pendant, since they could have hardly been worn in rings. Those which were worn in rings are flat bezels with a convex, concave or flat sealing surface, most frequently made of carnelian or sard. The fact that often the engraving of these stones is most effective if viewed against the light suggests that they should have been mounted on a movable setting. However, no such setting is preserved and the only extant rings with Sassanian seal-stones have the bezel solidly set into the metal. In addition to the red sard and carnelian, translucent chalcedonies and agates were most frequently employed for seal-stones. For the globular unfaceted seals, however, dark green jasper, flecked with red, and jasper breccia were also favoured. These colourful stones were highly polished and were in themselves lovely ornaments. Some connection may have existed between the shape of the seal-stone and the device of the engraving. This is especially true of the faceted and decoratively cut seals which frequently have a design of lilies on the base. The carving was done with much use of a mechanical drill, which produced globular forms. Details are usually indicated by lines which vary little in width and are judiciously applied to stress the main characterizing features.
The resulting globular appearance of the figures seems to have pleased Sassanian taste, since it conforms to a stylistic feature noted throughout the art of the period from reliefs to silver bowls. There is much variety in Sassanian seals, some of which are carefully engraved, others cursorily done; but the spacing is usually good and a certain assurance in the engraving differentiates the genuine seals from modern forgeries. The style may have originated in the eastern part of the empire, but at present it is difficult to speculate about a chronological and topographical classification of the material because the impressions of seals found in excavations have not yet been published. Most of these seals on clay bullae occur in groups centering around the large imprint of the seal of an official. Such official seals must have played an important role. According to Arab Writers Khusraw II (590-628 CE) had nine seals of state, and guarding the seals was an important office. From the impressions and extant seals we learn that some of the dignitaries merely had inscriptions engraved on their seals but others had busts, presumably of themselves and carved according to fixed conventions: the nose is indicated by a straight line and the nostril by a sharp curve, two pointed ovals form the lips, a long thin curving line renders the moustache, and short curving lines indicate the outline of the pointed beard and the hair, although this may also be shown by more rounded incisions. The eye is shown by a small ball set between the acute angle of the lids, over which arches the strong curve of the eyebrow. Usually the high officials who are portrayed by these busts also wear necklaces and ear-rings, the latter indicated by a minute globular pendant. Such carefully cut heads, however, are in the minority among the large number of those which are rendered in a very cursory manner.
Equally frequent as human heads or busts or palmettes are representations of animals. Of these the most common is perhaps the winged horse, seen in the same pose as the horses on the relief of Ardashir, with the neck bent in a semi-circular curve and one leg raised in a complementary curve. Even more than the human heads these horses are rendered according to a fixed scheme. Other animals often shown are stag, bull and ram, the latter wearing a necklace with the pleated ribbons of the royal diadem. The meaning of such renderings of a ram was probably connected with the auspicious royal radiance [xvarnah].
Another frequent group of designs shows either a female figure holding blossoms, a hand holding blossoms, or only one or more blossoms. The clue for the interpretation of all these seals is given by a more extensive scene in which a small female worshipper stands in adoration before a large female figure holding blossoms, surely the Persian goddess Anahita. We may therefore conclude that the hand holding blossoms, or blossoms alone, can stand for the same goddess and that part of her features and symbols can represent the whole deity. Unfortunately there are very few such carefully engraved scenes which would permit us to identify with certainty other gods and their symbols, the great variety of which cannot be adequately exemplified here.
A large number of Sassanian seals are decorated with so-called monograms. The simplest form consists of a moon crescent which tops a horizontal bar that terminates on either side in a curving hook. The precise meaning of these monograms is still unknown, though they have been associated with peasants’ house marks and also with objects resembling standards which are used by some nomadic peoples. Their occurrence in seals, however, surely indicates that they served one of the principal functions of the seal-stone, namely to protect the wearer and bring him good fortune.