Porosity, and what stone takes in
A great many statements about stone tools rest on a property that is rarely named. Serpentine takes dye readily. Jadeite has to be attacked with acid before it will accept polymer. Wax improves a polish on one material and does nothing on another. A wetted patch darkens on this stone and not on that one.
All four are statements about porosity, and it is worth defining properly, because once it is defined those become one fact rather than four.
Porosity, and the property that matters more
Porosity is the fraction of a material’s volume that is void rather than solid. Permeability is whether those voids connect to each other and to the surface, so that a fluid can travel through them.
The second is what governs everything of interest here. Glass contains bubbles and is not permeable at all, because each bubble is a sealed pocket with no route out. A recrystallised marble may contain less void space in total and be far more permeable, because its voids are the boundaries between grains and those boundaries form a continuous network. Total void volume tells you little; connectedness tells you almost everything.
Where the pore space is
Grain boundaries. In any rock made of many grains — marble, serpentinite, quartzite — the surfaces where grains meet are imperfect contacts, and they thread through the whole material. This is the principal pathway in most inexpensive tool stone.
Intercrystalline space in a fibrous fabric. Chalcedony and agate are masses of submicroscopic fibres with genuine micropores between them, which is why the family is the most dyeable material in the trade.
Fractures. An open crack is pore space with a wide aperture, and it is the fastest route into a stone. It is also why dye concentrates in cracks: they are where the liquid went.
Created porosity. Acid bleaching of jadeite dissolves material along grain boundaries and manufactures a pore network in a stone that did not have a useful one. That is the whole point of the step, and it is why impregnation follows it rather than preceding it — the entire Type B sequence is a porosity story.
And the materials that are effectively impermeable: a single crystal such as quartz or amethyst, with no grain boundaries to offer; glass and obsidian, whose voids are isolated; and the tightly interlocked fabric of good nephrite and untreated jadeite, dense enough that nothing enters at a useful rate. This is the actual reason those materials resist treatment — not hardness, which is a separate property altogether.
What follows from it
Dye uptake. A dye is a liquid, so it goes where liquids go: into connected pore space, preferentially into the widest apertures. The characteristic appearance of dyed stone — colour pooled along fine cracks and webbed along grain boundaries rather than sitting evenly in the body — is a direct map of the material’s permeability. The processes that produce it all depend on the stone letting something in.
Wax and oil. Wax fills surface pores, raising the apparent quality of a polish and deepening apparent colour, because a filled pore scatters less light than an air-filled one. On an impermeable material there is little for it to fill.
Staining. Anything that reaches connected pore space and then cannot be removed is a stain rather than a mark, and the same property that lets a stone be dyed lets it be stained by other things later. What to do about a stained tool is not this site’s subject, but the mechanism is: staining is evidence of permeability, and permeability is evidence about the material.
Strength. Pore space reduces the load-bearing cross-section and concentrates stress at pore boundaries, so a porous rock is generally weaker and less tough than a dense one of the same mineralogy. This is a general rule in rock mechanics rather than a precise prediction about any object.
Density. Void space is included in a bulk measurement, so a porous stone reads lighter than its mineral’s specific gravity — and takes water on during a submerged weighing, which is the reason displacement measurements are least trustworthy on precisely the porous materials.
The observation you can make
Water in a surface pore does something visible. Air scatters light strongly at a pore wall because its refractive index is nothing like the mineral’s; water’s is much closer, so light passes instead of scattering and the wetted area goes darker and slightly more translucent. As it dries, it returns.
A patch that darkens where a drop of water sits, and clears completely as it dries, indicates connected surface porosity — and therefore not a single crystal, not glass, and not dense jade.
The caveats are substantial and they run in one direction.
A polished surface is partly closed: fine polishing smears and compacts the outermost layer, and a waxed or coated surface seals it outright. So a piece that does not darken may be impermeable, or may be a porous stone with its surface sealed — a negative result establishes nothing. And introducing liquid to a porous stone is the mechanism of staining described above, so this is a test with a cost, not a free look.
What you can establish
WHAT YOU CAN TELL — about pore space
· Wetted patch darkens, clears on drying
→ connected surface porosity
· No darkening → impermeable OR sealed surface.
Establishes nothing
· Colour webbed along cracks and boundaries
→ dye, and a permeable material
· Slightly greasy feel, pits filled → waxed
· Water absorbed unevenly → variable porosity;
typical of a banded or veined rock
· How much pore space, or how deep → NOT
DETERMINABLE at home. A weighing procedureWhy the trade cares
Porosity is the lever behind most of what is done to inexpensive stone, and the interventions form a ladder rather than a category. At the bottom, wax in surface pores: a finishing step, traditional, generally disclosed and generally accepted. In the middle, dye in the pore network: a treatment, legal, frequently undisclosed by the time a listing is written. At the top, polymer through the bulk: a structural rebuild of the object, which is why it gets its own letter.
What unites them is that the stone had to be willing. The materials at the cheap end of the market are porous, and their porosity is not incidental to their being cheap — it is part of why they are workable, dyeable and abundant in the first place.
The limit
You can find out whether a surface admits water, which is a real if narrow finding, and you can read the dye pattern as evidence of a pore network. That is the extent of it.
You cannot quantify porosity, which is done by weighing a dried sample against a saturated one under controlled conditions. You cannot see past a sealed surface. You cannot tell created porosity from natural. And you cannot conclude anything from an absence: a stone that refuses water may be excellent dense jade or a well-waxed piece of marble, and telling those apart is a job for instruments and someone trained to use them.