I had a few stories swirling in my head about the process of finding these tools. The "search pattern" we develop in our minds based on experience and knowledge. Although before that it was the musing on how early peoples learned from observing the physical realty around them, (you know Mother Nature and all that).
I remember first finding examples of natural rock formations that screamed a challenge: you can do this! It was a little epiphany, all along nature was leading the way offering lessons to those who could see. Once people learned to process what they observed - there where these aha moments. Look at that pile of rocks over there, and why not stack them like that.
Or the occasional natural rock flakes that simply fit into someone's hand just right, someone who was curious, and heard that natural stone challenging, go ahead USE ME. From there, it's all imitation and getting better.