The oldest art falls into Class I (dated to the 6th through 9th centuries) and consists of primitive stone slabs (irregular shapes, un-smoothed surface) with simple geometric figures and some naturalistic figures (animals). Some shown here (see illustration) have no clear interpretation, and may have been used as boundary markers or to identify certain clans.
The designs were incised first with a simple rough punch and hammer, and then the line was widened and deepened with a chisel. The incised lines are smooth, so were likely rubbed with stone to eliminate roughness.
Second row, far-right in the illustration is what's called the "Pictish beast," but its identification is the source of much debate. Whatever it was, it had some significance since it accounts for 40% of all animal figures in Pictish stone carving. It has been described as a kelpie, a seahorse, a dolphin, or some monster; some have even suggested it represents the Loch Ness monster.
Class II stones are roughly rectangular and have symbols that are visibly linked with Christianity. They are often referred to as cross slabs because they depict crosses on one or both sides. They include Celtic geometric knot work as well. They date to the 8th and 9th centuries, so they started to appear prior to the disappearance of Class I carvings. Carving in Class II went beyond simple incising and brought the images out in relief.
Class III graduated to sculpture, in that the rectangular slab was not a canvas on which to carve a design: it was stone from which an actual shape was to be freed. Class III also abandons the pre-Christian symbols and lines, becoming similar to what was being produced by their Gaelic neighbors
The Picts also did some impressive metalwork, which I'll talk about next time.