Pre-cut presumably because they were smoothed by the water.
This. They've been sitting in the water for who knows how long.
All surface waterways might yield amber as well, with ponds having a slightly higher chance.
As far as I know, amber was only ever found washed up on ocean beaches--and while amber technically does exist in DF, no uses for it are implemented.
It seems to me that rough gemstones (or any stones of significant color) would more commonly be located at the bottom of waterways, not the shores. Meaning that the dwarves most likely to discover them would be not children, but Fish Cleaners.
The only reason I suggested waterways was presumably amber ending up dropping off trees near the rivers over time, and give you a reason to search the river banks for it, and use tropical oceans or lakes to collect coral chunks.
Additionally, the reason for making it along the riverbank is due to the issues with pathing a running river that's 7/7 deep causes. Easier to just consider say, three tiles out in any direction from the river to be considered searchable "riverbank," while oceans and lakes have their sandy beaches to sift through.
But that is a good point, you'd think you'd find a bit of amber, or a pearl, or bit of stone or gem swallowed by a fish by accident now and then.
As for stone and wood: rather than searching the ground for pebbles & sticks, it would make more sense to visit the Mason's and Carpenter's shops, take their "scraps" & "rubble" by-products, and use those. If not used for crafting purposes, 3 units of wood scraps can be burned as a single whole log, and rubble (of flux stones, anyway) can be ground into flux--all stone can be smashed into gravel for roadways and concrete.
It can be expected a lump of usable rock can sometimes be found having dropped off the ceiling of a cavern, tumbled down a hillside into the water, been rolled by the current or what have you, and usable hunks of wood floating downstream, large-ish branches having broken off trees in the rain and the like.
Still, those are good ideas regardless.
Randomly-scavenged bone would almost certainly be far too small for crafting purposes.
Hence only single units. Bits of fish bone or bird bone left washed up on the river bank that might be usable for some kind of decoration or a bolt tip, if nothing else. May even make a difference for some dwarf if you keep getting animals too small to butcher roaming in and thus no bones.
Embarks that already have access to shells don't exactly need more shells. What would be far more useful is for forts with no such access to have a way to import them. Currently, all imported clams, cave lobsters, turtle meat, etc., is shell-less.
It seems more like this is meant for places lacking in such things in great amount or variety, and could potentially be a life save if your fisherdwarves insist on pulling in cave fish and trouts instead of a mussel or cave lobster your modding dorf needs.
Pre-cut presumably because they were smoothed by the water.
All right, that's a good enough approximation for DF, but I still object for 2 reasons:
1. If a small rock is shiny enough to be immediately used as a gemstone, it's shiny enough for a bird to steal it away to decorate its nest (which currently don't exist).
2. Dwarves are supposed to be renowned for their artistry, and craftwork par excellence. Are we now literally just picking trash up off the ground and saying, "Here, buy dis"?
1. It depends on who gets to it first. Screw the birds, it's our shiny crap now!
2. You seem to think dwarves wouldn't have a few hucksters among their ranks. Or Gem Setters who don't want to wait for a cutter to do his job.