The gravel came from somewhere close by.
It doesn't pay to ship driveway gravel too far. Sand and gravel quaries are often aluvial deposits so the rocks in them might have come from somewhere far away.
Other people have posted pics of agates found in their gravel.
I have found small terminated quarts crystals in gravel that comes from a couple of miles behind my house.
Now that you know what to look for you should find more.