Flint and Chert Rock

I do believe your examples are composed of flint and chert stone. Both are natural materials with a long history of use by humans. They are both relatively common, and can be found in many parts of the world.

They are both formed when silica, a compound of silicon and oxygen, is deposited in layers. The layers of silica can be very thin, which is why they have a microcrystalline structure. Both are often found in sedimentary rocks, such as limestone and chalk. Flint is typically darker in color than chert, ranging from gray to black. It can also be found in a variety of other colors, such as brown, red, and yellow. The dark color of flint is caused by the presence of carbonaceous matter, or organic material. Chert is often white to pale brown or gray. It can also be found in a variety of other colors caused by the presence of water or air.

These are both very hard and durable materials. They are also very brittle, which means that they are more likely to break into sharp pieces when they are struck. This makes them ideal for making tools and weapons, such as arrowheads, spear points, and knives.

A mammal hoof will fail to fossilize because of its organic composition. The difference between bone and keratin is keratin breaks down easily when exposed to air, moisture, and scavengers. It lacks a hard structure where in fossilization mineralized parts like bones, teeth, or shell can withstand the elements long enough to be buried and filled with minerals. This is unlike soft tissues and keratin structures that vanish long before sediment can seal them away from decay caused by microbes.