When your thumbnail just isn't strong enough
Artificial nails were in use many tens of thousands of years ago...
Just not the ones that normally come to mind, when that phrase is used.
I introduce to you the Thumbnail scraper.
When you have a bit of bark that need peelings, or a piece of leather, that needs a good scrape, to clean it up and soften it. Don't break a nail, use a thumbnail scraper...
Scrapers are made from broken blades and any other suitable stone flakes.
The edge of the flake is pressed against a hard stone to break off fine chips and steepen and strengthen the edge.
The flake or broken blade is held in the fingers as shown below.
The steepened and strengthened edge then provide a fine scraping tool...
The smallest ones are indeed the size of a thumbnail and these would be used for fine, delicate scraping tasks.
Generally for things like the final stages of softening fine leather, trimming bows and arrows, preparing food etc.