I am going to try demonstrating that some old rocks are actually deliberately manufactured stone tools - badly weathered but still identifiable.
First, consider the percussive flake - a concave facet, created by force focused along the edge of a rock. When a rock is struck with force, a cone of energy propagates through the rock and can break off a piece, as a flake. You can wonder how nature might produce these (when rocks fall off a cliff, or are thrown to land by a big wave) but for the most part these can only be created by man. If you have been seeing them your whole life it is because you have been seeing stone tools - they are very common. In some places, for example in the desert, it can be hard to find any rock that has not been percussively flaked sometime in the past.
With nice glassy volcanic materials, percussive flakes usually come along with concoidal fracture, and there are other significant details. But on coarse grained materials the flakes do not look so good. Here are some flakes in a pretty good material:
Note the straight lines where the flake ends early along the central ridge. These are called "hinge" flakes - places where the natural grain of the rock frustrates the conic section and a kind of step is produced.
Here are flakes made in slate, which has natural cleavage planes:Here are flakes in quartz, which are entirely based on cleavage planes, all hinge all the time:Generally, if a percussive flake is present, the stone was worked. More specifically, when a sequence of similar size flakes occur - especially when they alternate in directive (front/back/front/etc) then this is strong evidence it was not just broken but broken in order to produce an edge.
Flakes are harder to recognize but are no less unique, when the rock has been severely weathered. The edges of the flake get rounded and, with enough wear, the flake becomes a dimple on the rocks surface. At that point it is only the sequence of flakes (now dimples) that can attest to the object having been made deliberately.
Here are three stones I want to compare:(from the other side:)
From left to right, these are slate from Shirley, basalt from Wilmington parking lot gravel, quartz from the Nevada desert. Based on flaking alone, the slate and quartz are obviously worked. You must put your faith in dimples to believe the middle item was deliberate - it has been through a glacier and had lots of iron deposited on its surface.
But based on a higher order concept of design comparison, there is less doubt these three objects represent the same kind of tool. Following the principles discussed here we must define the design and then discuss how these objects fit that design. We build upon the flake "design concept", assume it needs no further demonstration, and apply the principle that complex designs are demonstrated if they are composed in the same way from the same simpler design elements.
So, here is the proposed design:This is a pretty faithful description of the shape you see three times in the above photos. Also, following the principle that common design features have common function, it is fair to say this tool type included: some kind of neck for hafting and some kind of sharpened edge and tip. One can add that the hafting was asymmetric.
The slate example is the best preserved and examining the "tip" and sharpened edge, one sees the flakes are relatively fresh on one side of the long-axis midline, and are worn down on the other side of the midline. In other words, the lower right hand side of the above design was the functional edge. This gives us a final guess as to the nature of these objects. Something shaped a bit like this:I suppose this is close to nonsense. But I believe this shape actually represents a very persistent design that was present throughout the US, somewhere back around or before the last glacier.