When your Wood Crafter takes a log big enough to make a coffin out of, and instead produces a ring, an amulet, and a figurine, what happens to the rest of the wood? Even when your Carpentry shop is running full-blast, churning out doors and beds, you're bound to have some leftover scraps.
I propose that jobs such of these use the relative size of the finished product(s) and use that to calculate how much of the principal reagent is left unused--but still theoretically usable. Each workshop keeps track of 1/10ths of logs (just as Smelters do with 1/10ths of bars), and, when there's enough for a full "log", out pops 1 unit of firewood (pretty much all woods are functionally identical, so I see no need to differentiate). This wood is good for nothing but the Wood Furnace. Appropriate workshops would also have the ability to destroy finished wooden objects (which you might want to do if they're xXdamagedXx), creating similar fractional logs--although if the objects have been further embellished by material decorations, those decorations may be lost, depending on their composition.
Gravel would be similar, but rather more complicated. Stonecrafting and Masonry jobs would result in waste gravel, but the main source of it would be mining: every mined rock tile would generate some gravel, let's say 1/10th of a stone, which would clutter up the tunnels until it was hauled away in bags or barrels. Gravel could be used as cheap road material, a means of filling aqueducts to make them water-permeable but impassable by most creatures, a prime component of mortar (why doesn't the game have mortar yet?), or simply to fill holes. The problem lies in keeping track of the various types of gravel: Flux gravel seems a given, as realistically even 1/10th of a load of flux stone would be ample for both steps of a complete steel-making process. Gravel of brightly or richly-colored stone, like cobaltite or orthoclase, would be rightly prized for use in mosaics or decorating pottery . . . but that would mean keeping track of the root stone type of every tenth of every unit of gravel (at least, until it gets used--I don't really see dwarves deconstructing a gravel road to salvage the gravel for future use), and that seems unnecessarily cruel to one's FPS.