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Types of Natural Whetstones

As we start to dig into natural whetstones, identifying the type of geological formation the stone comprises of can help to estimate the techniques which work best with its use. While no two stones are exactly alike, two stones which are formed the same way will be far more similar than two stones which are geologically different. An important note here is that nature does not divide along the same clear-cut lines that humans prefer. Many stones can be more like a “mix of two different types” of stone rather than one specific stone. The formation of stone can stop in the middle of the two ends of the metamorphic process and sometimes create middle grounds stones which are hard to classify firmly. Many different types of stone can be used for natural whetstones, however the ones we most readily used break down into 4 different categories:

Shale Types of Natural Whetstones

This fine-grained sedimentary rock is formed from compact silt, clay, and marine plankton life fossils into what is known as “mudstones”. Unlike other mudstones, proper Shale stone is compact enough to where it is laminated (having many distinguishable layers) and fissile (the rock readily splits along the laminations).

Some common examples are below:

Shale’s diverse range of composition and mineral inclusions though make this a very wide range from 2.2-2.8 SG with many falling in the lower end of the range.

Shale natural whetstones are most commonly used with slurry on the surface of the stone which is sometimes called “mud”. These stones are not suitable for use with oil as their loose composition will hold the oil and become clogged.

Novaculite Types of Natural Whetstones

This is a dense, hard, and fine-grained siliceous rock which is distinguishable from other formations by its nature to conchoidal fracture (when it breaks it does not break along any natural separations like how glass breaks). Novaculite started its life as Shale specific to ancient marine environments where organisms such as diatoms (algae that secrete a hard shell composed of silicon dioxide) were abundant. This shale was then metamorphosed via a heat/pressure/chemical process which crystalized its silicon dioxide forming Chert. This Chert stone was then further metamorphosed where the crystalized silicon dioxide was changed into microcrystalline quartz grains.

Some common examples are below:

Novaculite’s specific gravity range is usually within 2.2-2.7 SG. Specific gravity in the 2.65+ range for Novaculite can be undesirable as it usually indicates a lesser percentage of pure quartz crystals which has a SG of 2.65. The lower the specific gravity of Novaculite to 2.65 indicates a less compact stone (such as Washita) which will act coarser comparative to ~2.65 Novaculite stones. If the stone has been used with oil it will skew the results up.

The formation of Novaculite lends itself to different techniques than that of Shale or Slate. Because its silicon dioxide is in the form of quartz crystals, these crystals come close to or in the higher SG range form what is referred to as a matrix. This is where the stone is a compressed matrix of crystals which have all grown into one another and are not suspended in a binder material like other stones.

Due to this, most Novaculite stones create very aggressive slurry because it is chunks of crystal rather than suspended bits of non-crystal silicon being sheered from the stone when the slurry is formed. These small crystals in your slurry will readily abrade the edge of the blade but at a level far coarser than the stone is typically advertised at. Usually, this is so aggressive that you will not then be able to finish the blade on the same Novaculite stone. It may be possible with some Novaculite whetstones to generate a slurry and slowly dilute it overtime, but this is an outcome of how the stone was formed and its density. Slurried Novaculite can still have its place when working blades, but it will not function like most Shale or Slate stones where you work slurry on the stone up to finishing.

With the above in consideration, most Novaculite stones are used as what we can refer to as “finishers” where you have worked the blade on a series of stones and now want to finish with the keenest and final step. To accomplish this, most people will condition the surface of the Novaculite stone to not only be flat, but very smooth (1000 ANSI Grit/2000 JIS Grit). You can then use water, a water/glycerin mix, or oil on the surface to finish the blade to a very fine level. Even without slurry and finished to such a smooth level, the surface being mostly quartz crystals will still greatly impact the blade edge in a positive way.

Slate Types of Natural Whetstones

Some common examples are below:

Are often very hard and slow and can be used with water or oil. Oil often helps bad technique or an aggressive stone to finish.

Schist Types of Natural Whetstones

Some common examples are below:

Are often very hard but usually cut pretty fast and can be used with water or oil. Oil often helps bad technique or an aggressive stone to finish.

Stones outside of these categories like chert, marble, quartzite, etc. can also make whetstones but often lack inherent qualities we find valuable in the aforementioned four which makes them preferrable.

Jasper/Jade/Agate Types of Natural Whetstones

Some common examples are below:

  • Apache Whetstones

Jasper and Agate are actually visual terms rather than formation terms. These are almost always some type of Chert, and are similar to Novaculite but of a far lesser quality of the purpose of whetstones. Their crystalline formation is much less consistent, and often they will have “soft spots” within their structure. This results in the honing surface impacting the blade unevenly. Even if the stone does not have any soft spots, the structure of the stone is extremely hard and offers issues when sharpening blades free-hand. Jasper/Jade/Agate with no soft spots can be acceptable for razors (since the likelihood of rolling the edge is gone due to the guiding spine) but it won’t offer much other, better, and more versatile options do.

Outside of a few one-off examples, Jasper/Jade/Agate has never been used as a commercially successful whetstone primarily because of the aforementioned problems it has. You can still find them though available for sale (a large market sells them in “pendant” or “Viking pendant” format) but most will find their time better spent on other stones.