Quote I'm reminded of, from Matthew Crawford's Shop Class as Soulcraft -- long but I think worth reading:

Following graduate school in Chicago, I took a job in a Washington, D.C. think tank. I hated it, so I left and opened a motorcycle repair shop in Richmond. When I would come home from work, my wife would sniff at me and say “carbs” or “brakes,” corresponding to the various solvents used. Leaving a sensible trace, my day was at least imaginable to her. But while the filth and odors were apparent, the amount of head-scratching I’d done since breakfast was not. Mike Rose writes that in the practice of surgery, “dichotomies such as concrete versus abstract and technique versus reflection break down in practice. The surgeon’s judgment is simultaneously technical and deliberative, and that mix is the source of its power.” This could be said of any manual skill that is diagnostic, including motorcycle repair. You come up with an imagined train of causes for manifest symptoms and judge their likelihood before tearing anything down. This imagining relies on a stock mental library, not of natural kinds or structures, like that of the surgeon, but rather the functional kinds of an internal combustion engine, their various interpretations by different manufacturers, and their proclivities for failure. You also develop a library of sounds and smells and feels. For example, the backfire of a too-lean fuel mixture is subtly different from an ignition backfire. If the motorcycle is thirty years old, from an obscure maker that went out of business twenty years ago, its proclivities are known mostly through lore. It would probably be impossible to do such work in isolation, without access to a collective historical memory; you have to be embedded in a community of mechanic-antiquarians. These relationships are maintained by telephone, in a network of reciprocal favors that spans the country. My most reliable source, Fred Cousins in Chicago, had such an encyclopedic knowledge of obscure European motorcycles that all I could offer him in exchange was regular shipments of obscure European beer.

There is always a risk of introducing new complications when working on decrepit machines, and this enters the diagnostic logic. Measured in likelihood of screw-ups, the cost is not identical for all avenues of inquiry when deciding which hypothesis to pursue. For example, the fasteners holding the engine covers on 1970s-era Hondas are Phillips-head, and they are always stripped and corroded. Do you really want to check the condition of the starter clutch, if each of ten screws will need to be drilled out and extracted, risking damage to the engine case? Such impediments can cloud one’s thinking. Put more neutrally, the attractiveness of any hypothesis is determined in part by physical circumstances that have no logical connection to the diagnostic problem at hand, but a strong pragmatic bearing on it (kind of like origami). The factory service manuals tell you to be systematic in eliminating variables, but they never take such factors into account. So you have to develop your own decision tree for the particular circumstances. The problem is that at each node of this new tree, your own, unquantifiable risk aversion introduces ambiguity. There comes a point where you have to step back and get a larger gestalt. Have a cigarette and walk around the lift. Any mechanic will tell you that it is invaluable to have other mechanics around to test your reasoning against, especially if they have a different intellectual disposition.

My shop-mate Tommy Van Auken was an accomplished visual artist, and I was repeatedly struck by his ability to literally see things that escaped me. I had the conceit of a being an empiricist, but seeing things is not a simple matter. Even on the relatively primitive vintage bikes that were our specialty, some diagnostic situations contain so many variables, and symptoms can be so under-determining of causes, that explicit analytical reasoning comes up short. What is required then is the kind of judgment that arises only from experience; hunches rather than rules. There was more thinking going on in the bike shop than in the think tank.

Oooh yeah dude I think you want to un-buy the obsidian. That is almost literally glass. You want chert!

Lots of animals have problems with their nails and claws

Are you thinking of pets, or also wild animals? I figured part of the problem was that wild animals (and our ancestors) wore down their fingernails and toenails at approximately the same rate that they grow, so they would stay in a good configuration and everything was fine, whereas pets & modern humans have more nail problems because we under-use our nails, grow them out for aesthetics, etc. (But I could be wrong!)

There were probably a huge number of wooden tools that archaeologists have never seen because wood decays.

I know how this works because my father made a telescope mirror in our basement. (He had to get it silvered professionally, but he did all the glass grinding/polishing himself!)

Are there some people who do this for a hobby?

I was first exposed to it (flintknapping, but you can also make stone tools by grinding stone) by a granduncle when I was very young (5), and did it as a hobby for the last ~1.5 years of my undergraduate degree while I was living with my parents. I don't do it anymore because I live in an apartment and don't have easy access to a space where I could do it. It takes a lot of outside area because you produce a lot of sharp stone flakes and silica dust which you don't want in an area that is not well-ventilated.

You probably need the right kind of stone for that. How much time does it take for a relative beginner to produce one stone tool?

It doesn't work with any rock. You need materials that produce conchoidal fractures. The most typical ones to work with are flint, chert, obsidian, and dacite. I'm not a geologist, but what is referred to as "dacite" in the flintknapping community doesn't look a lot like what I see in the dacite Wikipedia article or Google Images (has a lot of inclusions, doesn't look like it would fracture the right way). If you look up "flintknapping dacite" you'll see it as a slightly glossy dark grey rock. More exotic materials are fossilized coral ("agatized coral") and quartz crystals (https://www.zmescience.com/science/archaeology/rock-crystal-dagger/). Non-natural materials can be used as well. Some people knap with non-natural glass and I think porcelain.

There are several websites that sell the rocks you need for very cheap if you don't live in an area where you can find them. They also sell traditional tools. The typical tools are other rocks (different kind) and antler. Sometimes copper is used, and I think it was used historically during the chalcolithic.

As to the difficulty of knapping, it depends heavily on the material. Obsidian (and by extension, probably manufactured glass as well) is by far the easiest of the materials I listed, followed by dacite. With the low level of skill I reached, I was only ever able to make decent looking tools out of obsidian and dacite. You need to strike the rocks very precisely to knap well, but for flint and chert you need to hit them a lot harder, which makes precise hits tougher IMO.

You can knap decent looking tools out of obsidian with not too much practice, and it is an extremely fun hobby that I would recommend quite highly. I was able to make do with YouTube tutorials and produced rough-looking blades with less than a week of practice (probably <10 hours). You'll break a LOT of blades while learning though, which really sucks if they were looking nice up to that point. You'll also cut yourself, especially if you don't wear gloves. The cuts are generally very clean and heal nicely, especially with obsidian. I wear glasses for my vision, but I would probably wear protective glasses otherwise. Apparently Ishi (https://en.wikipedia.org/wiki/Ishi) had a technique for dislodging shards of the material from his eyes (https://archive.org/details/yahiarcherysaxton00poperich/page/116/mode/2up?q=eye). 

See Primitive Technology on Youtube. 

Epistemic Status: Memory. I learnt this (weirdly) in an astrobiology course at University. And through long personal interest in prehistory.

I know that the stone heads themselves are flint. To get it into the shape you want, you have to hit the flint with a harder rock. You hit it for (I think) hours^[1] , in precise places around the edges. Inexperienced people may hit the flint too hard, when it's nearing completion, shattering hours of work, and requiring the process to start again. I have heard learning how to make good spear heads can take quite a long time.

1. ^{^}

   [Edit: I think I was wrong about the Hours thing. In David Miles' "Tale of the Axe" he claims it takes ~20 minutes for an experienced individual to Knapp a flint hand-axe.]

Me too!  I believe that evolution DID fix it--apes don't have this problem--and that the scrotum devolved after humans started wearing clothes.  'Coz there's no way naked men could run through the bush without castrating themselves.

I'm not sure if you understood this or not, but, I think Alex meant "nutrient" in a metaphorical way rather than a literal dietary way.

Knitting is an interesting example, as it's a relatively recent invention (less than 1000 years old). Previously to make fabric you needed some kind of weaving method, which requires some kind of structure to hold the fabric. With knitting, you can do it anywhere, and just need a stick or two. It's an interesting technology in how important it was, in a very subdued way. Knitting uses more materiel, but you get a flexible and warm garment which you can produce while on the go (apparently Chinese caravan men would pluck out camel hair to continue knitting).

It's also interesting in how quickly it was abandoned as soon as cheap fabric was available from other sources - knitting is what grandmothers do (this used to make a lot of sense) and so was low status. It's only recently become a normal hobby.