River rock sharpening stone

I used to buy fancy sharpening stone made in China - not realizing there are free one available in the river. These are the stone I found in the river by chance and it's harder than the China made sharpening stone. I tried to cut them using metal hand saw and not a bit of scratches on it. I have tested it's pretty good to sharpen Swiss Army knife. Our mind is too confined with what is available in the shop not realizing sometimes nature has provided them for free.