Q: I was told that as a beginning carver I should avoid stone that has a lot of colored bands or layers. Why? Is colored or banded stone more difficult to carve?
A: There are two primary aspects to consider when working with banded or layered stone. One aspect is concerned with esthetic or artistic character, the other is with a stone’s structural and carving properties.
Parallel or nearly parallel bands may be: 1. sedimentary bedding, 2. metamorphic foliation, in which minerals have recrystallized as parallel crystals or in separate layers of different minerals, or both, 3. colored layers from chemical alteration by fluids moving through rock and depositing mineral material, or 4. several other possibilities as well. The banding may be microscopic, a fraction of an inch, or many feet wide.
I don’t pretend to be an arbiter of esthetic or artistic taste, but I suggest you consider the scale and subtlety (or lack of it) of a banded stone in relation to the piece you plan to carve. A conspicuously banded stone can overwhelm a complex shape or detailed carving. The viewer may see only the color pattern and not the detail. But a highly figured stone may work very well with broad abstract forms. Color banding can generate a fascinating variety of patterns depending on the shape and orientation of the carved surfaces. It becomes a matter of taste - yours.
Carving considerations are somewhat less subjective. Banding, no matter its origin, may be a direction of weakness. Until I am familiar with how a stone works I won’t attempt to carve a thin horizontal projection, an extended arm for example, from a stone with the banding oriented vertically. Familiarity is the key. Test the rough stone with a point or chisel parallel to the banding and at various angles to it. Does it split easily with the banding? Do you generate flat chips parallel to the banding? If so, you may want to reconsider your design or choice of stone.
Not all color banding is structurally unsound nor is all unbanded stone necessarily uniform. Onyx marble, a very fine-grained translucent variety of limestone, is often strongly color banded but it exhibits no preferential breakage along the bands. At the other extreme is slate, a uniform-appearing metamorphic rock composed of very fine flat parallel crystals of various minerals. Carving across the grain is like carving many other stones, but a light tap with a chisel parallel to the crystals can cleave off a sheet a quarter inch thick and a foot square: a slate roof shingle.
Many stones, layered or not, carve differently in different directions, much like wood grain but often more subtle. Paying close attention to how a stone carves in different directions will help you determine any weaker direction, no matter whether you use hand tools, air tools, or even rasps and rifflers.
Was the advice given you justified? Asking for a direct answer from an expert (and geologists are among the worst) usually results in “It depends.” And so it does. Use your judgment for esthetics and your point, chisel, and courage for structural considerations.