Reading a polished surface

The polish on a stone tool is evidence about two different things: the material, because different minerals take a polish differently, and the finishing process, because every stage of grinding leaves a signature. Held at a low angle to a bright light, a polished face shows features that are invisible when you look straight at it. Several of them are more informative than any test you can run.

Look along it, not at it

The technique is simple and it is the whole method. Hold the piece so that a bright light source reflects off the surface toward your eye at a grazing angle, then rotate it slowly. What you are looking for is not the reflection but its distortions: where the reflected image of the light source stretches, breaks, dimples or dulls.

A flat, well-polished surface returns an undistorted reflection. Everything else is a departure with a cause.

Undercutting

The observation. The surface is not quite flat. As the reflection sweeps across it, faint relief appears — a fine texture of slight hills and hollows, sometimes with a directional or fibrous character.

The cause. The material is an aggregate of crystals that differ in hardness, or of one mineral whose hardness varies with crystal direction. During polishing, the softer or unfavourably oriented grains abrade faster and end up slightly below their neighbours. The polishing wheel cannot make a mixed-hardness surface truly flat.

What it establishes. That the material is an aggregate, which is a real structural finding. Single crystals and glass polish uniformly because there is nothing in them to abrade at different rates. This is the surface expression of structural class, and it is often easier to see than fracture behaviour, since it needs no damage.

In fibrous nephrite the relief may follow the fibre direction. In granular jadeite it produces a dimpled texture known in the trade as orange peel, a recognised feature of well-polished jadeite rather than a fault.

Pits and cavities

The observation. Small holes or depressions in the surface, sometimes with the polish continuing into them and sometimes not.

The cause. Two possibilities, distinguishable. A cavity is where an inclusion or a soft patch was removed during working, or where the rock was porous — natural, and the polish carries into it or stops at a natural boundary. A pit from incomplete finishing is a leftover from a coarser grinding stage that the later stages never reached, and it has the dull matte texture of the coarser step.

What it establishes. Natural cavities indicate a porous or heterogeneous material. Grinding pits indicate a rushed finish, and where they persist you can often see the coarse-stage scratch texture inside them, which tells you the shortcut was taken.

Scratch fields and their direction

The observation. Fine parallel lines, all running the same way, usually visible only at a grazing angle.

The cause. A grinding or sanding stage whose marks were not removed by the following one. Lapidary finishing works through progressively finer abrasives, each removing the previous stage’s scratches, and skipping or shortening a stage leaves them. How the sequence works explains why they are parallel and why they are directional.

What it establishes. The quality of the finishing, not the quality of the material. These are different things and conflating them is the commonest misreading of a surface. A fine stone can be badly finished and a poor one beautifully finished.

Distinguish these from use scratches, which are random in direction, varied in depth, and concentrated where the object gets handled. A soft material such as serpentine accumulates them quickly.

Seams, flats and the shape of the outline

The observation. A faint line running around the piece, a slight ridge where two curved surfaces meet, or a facet on what should be a continuous curve.

The cause. A mould parting line if the object was cast — glass or resin. A facet on a curve is usually the mark of hand-grinding against a flat wheel, or of a tumbling process that rounded the piece unevenly.

What it establishes. A parting seam is strong evidence of a moulded product and therefore of glass or resin rather than stone. Ground facets are evidence of the working method, not of the material.

WHAT YOU CAN TELL — from the polish, at a grazing angle

  · Faint relief or dimpling on a flat face
      → aggregate with differing grain hardness
  · Perfectly uniform mirror surface
      → single crystal or glass
  · Parallel scratch field in one direction
      → unfinished grinding stage; workmanship only
  · Random scratches concentrated where handled
      → use wear; suggests a soft material
  · Faint line running around the whole piece
      → mould seam. Suggests a cast object
  · Whether the surface carries wax or polymer
      → NOT DETERMINABLE by eye. Needs infrared
        spectroscopy

Why a surface can look better than the stone

The commercially important point about polish is that it can be substituted for material quality, and the substitutions are cheap.

Wax fills. A thin surface coating of wax fills microscopic pits and fine surface-reaching cracks, raises apparent lustre and deepens colour. It is traditional and accepted in the jade trade, and it also means the surface you are examining may not be the stone’s own.

Coatings do more. Lacquers and deposited films go further, and their tell is at a chip rather than on the face.

Tumbling hides geometry. A barrel-tumbled piece has soft, rounded, slightly uneven edges that disguise both crude shaping and small chips.

So a very high polish on an inexpensive object should prompt the question of what is producing it. A genuinely well-polished aggregate almost always shows some undercutting; a flawless mirror finish on a green stone is more consistent with glass, resin or a coating than with an unusually good piece of rock.

The limit

Surface reading establishes structural class and workmanship, and it catches mould seams. That is more than most people expect from an observation requiring only a lamp.

It does not establish species, and it cannot detect the surface treatments that most affect what you are seeing — wax and polymer at the surface are invisible to the eye and detected by infrared spectroscopy. As with everything else here, the surface eliminates and suggests without confirming, and the most confident thing you can say from a polish alone is whether the material behaved like an aggregate under the wheel.