Stone Types
Part of Mill Construction
A guide to the geological types of stone suitable for millstones — how to identify them in the field, their relative merits, and how stone type affects mill operation.
Why This Matters
Not every hard rock makes a good millstone. The ideal millstone material balances hardness, toughness, and porosity in a specific way that no single geological description captures completely. The same “granite” from two different locations may perform very differently. Understanding the geological context of stone — how it formed, what minerals it contains, how those minerals behave under repeated impact and abrasion — allows you to select the best available material in your region rather than simply quarrying the nearest hard rock.
This knowledge also helps predict how a stone will perform over time. A miller who understands why their granite stones need re-dressing more often than French burr stones (different wear mechanisms) can plan maintenance more effectively and adjust operating practice to get better results from inferior materials.
In most rebuilding scenarios, you will not have access to the historically preferred stones. But every region has rock formations, and most regions have at least one stone type that can be made to work. The skill is in finding and evaluating it.
Siliceous Rocks (Best Category)
Siliceous rocks — those dominated by silicon dioxide (quartz) — make the best millstones because quartz is very hard (Mohs 7), chemically inert, and doesn’t contaminate flour. But pure quartzite is actually too hard — it doesn’t dress well and wears the miller’s bills rapidly without cutting effectively. The best siliceous stones have a more complex texture.
Freshwater silica (burr stone): Formed in ancient lake deposits, this material consists of angular quartz grains in a siliceous cement, with an open, porous texture from the gas bubbles and organic material in the original sediment. This porosity is the key — it maintains a self-sharpening cutting action as the stone wears. French burr from La Ferté-sous-Jouarre is the most famous example, but similar formations exist in the silicified lacustrine deposits of many regions. Look for chalky-white or yellow-white rock in areas with ancient lake beds, with a cellular or spongy texture, harder than limestone but not as dense as flint.
Chert and novaculite: Dense, fine-grained siliceous rocks formed from silicified marine sediments. Chert (also called hornstone) is extremely hard and very tough — more resistant to cracking than most other stone types. It is less porous than burr stone and may need more frequent stitching to maintain its cutting action. Arkansas novaculite (a fine-grained chert) was the traditional American whetstone material and makes an excellent millstone where available.
Quartzite: Metamorphosed sandstone in which quartz grains have been re-cemented with silica into an extremely dense, hard rock. Too dense for the best grinding — the working surface glazes over quickly — but acceptable if nothing better is available. Dress frequently and use for coarser grinding applications.
Volcanic Rocks (Very Good)
Vesicular basalt (lava millstone): Volcanic basalt with trapped gas bubbles creates a naturally porous texture ideal for millstones. The best-known variety is “Cullin” from the Eifel volcanic region of Germany, but similar vesicular basalt occurs wherever volcanic activity has occurred. Look for basalt (dense, dark gray to black, very hard) with a texture that shows tiny pores visible to the naked eye. The pores must be fine and evenly distributed — coarse voids mean the stone will crumble at the surface.
Vesicular andesite: Similar to vesicular basalt, slightly less dense. Where the volcanic geology includes andesite (intermediate silica content volcanic rock), it may make excellent millstones.
Pumice and scoria: Too light and porous — they break down too quickly under milling pressures. Not suitable.
To identify vesicular volcanic rocks in the field: they are typically dark colored (gray to black), heavier than limestone, and may show a glassy or crystalline texture mixed with porous zones. They ring distinctly when struck with a hammer (compared to the thud of limestone).
Sandstones (Good for Many Applications)
Millstone grit: Coarse-grained sandstone in which individual sand grains (quartz, 0.5–2mm diameter) are loosely cemented, giving the surface a naturally rough grinding texture. The grains are hard enough to cut grain but shed gradually in use, constantly exposing fresh cutting surface — a self-sharpening effect similar to burr stone, though less pronounced. The best millstone grit has evenly sized quartz grains with a moderately strong cement. Too weak a cement and the surface crumbles; too strong and it glazes.
Testing sandstone: scratch the surface with a fingernail — you should be able to dislodge individual grains. The grains should be distinct and quartz-like (glassy, translucent), not calcite (which will effervesce with a drop of acid).
Sandstone millstones produce acceptable flour but require more frequent dressing than burr stones and the flour may be slightly gritty until a new stone is broken in and the initial loose surface grains are cleared. Run new sandstone stones for a few hours grinding waste material before using them for food flour.
Granites (Acceptable)
Fine-grained granite: Granite consists of three minerals — quartz (hard), feldspar (medium hardness, weathers to clay), and mica (soft). Coarse-grained granite is problematic because the large feldspar crystals weather out during use, creating soft spots and grit contamination. Fine-grained granite (where individual mineral crystals are less than 1mm across) behaves much more uniformly and can make serviceable millstones.
Look for fine-grained granite or granodiorite with an even, speckled gray texture and no visible large crystals. “Blue granite” (actually a fine-grained gneiss or granodiorite) is much better than typical pink or gray coarse-grained granite.
Granite stones run hotter than siliceous stones because the harder surface creates more friction. Monitor flour temperature carefully when first running granite stones.
Limestone and Marble (Poor to Unacceptable)
Limestone is calcite — much softer than quartz (Mohs 3), reactive with the acids in grain, and prone to contaminating flour with calcium carbonate. Limestone millstones wear rapidly, require constant re-dressing, and produce gritty flour. Use only as a last resort.
Marble is metamorphosed limestone with the same drawbacks, plus an even harder-to-dress surface. Not suitable.
Test with acid: a drop of vinegar or dilute acid on limestone produces vigorous bubbling (carbon dioxide). Good millstone material should show no reaction to acid.
Field Testing Procedure
When evaluating a potential stone source:
- Acid test: Drop vinegar on the surface — no bubbling confirms absence of carbonate
- Nail scratch test: Try to scratch the surface with a steel nail — a good millstone material leaves no scratch
- Hammer test: Strike with a hammer at several points — a consistent ringing note indicates uniform composition; dull thuds indicate soft zones or voids
- Fracture test: Break off a small piece and examine the fresh fracture — it should be sharp, angular, and consistent in texture across the full fracture face
- Porosity test: Wet the fracture surface and observe how quickly it dries — moderate porosity (dries in 30–60 seconds) is ideal; stone that stays wet for minutes is too dense; stone that is already dry before you finish wetting it is too porous
Take several samples from different parts of the outcrop and test all of them. Stone quality can vary significantly across even a small outcrop, and you want to quarry from the most consistent zone.