Since the development of the rifled barrel, just about five centuries ago, rifle stocks have been made out of materials from wood to space-age synthetics. They've also included materials not essential to rifle function-unless a shooter considers impressing their friends and depressing their enemies essential-including inlays of gold, ivory and gemstones.
Wood dominated stock materials for most of the past half-millennium, for several reasons. It's relatively abundant and "renewable," and can be easily shaped with both hand and power tools to fit what was originally known as the lock and barrel, and nowadays called the barreled action.
Hardwoods dominated on more powerful rifles due to recoil resistance, with walnut the most common. European walnut (juglans regia) is considered the best hardwood by many of the dwindling number of wood addicts, because it tends to be lighter in weight than American walnut, yet stronger, a good combination for rifles carried very far into the field. It also has less tendency to split and splinter when cut, and its smaller pores make the surface easier to finish.
Other woodophiles consider American walnut more beautiful than European, especially Claro walnut (juglans hindsii), native to northern California. Claro grain can have spectacular contrasts and colors, but the wood itself tends to be softer than other walnuts. Juglans nigra (literally black walnut) of eastern North America is considered easier to checker and inlet than Claro, though not as easy as European walnut. Both American walnuts also have larger grain than European walnut, making them harder to finish--unless simply covered in a relatively thick layer of modern synthetic "varnish," somewhat obscuring the grain. Still, black walnut's used on many American factory rifles, especially plainer, straight-grained black walnut. It grows relatively fast, so doesn't cost as much as rarer Claro or imported European walnut. (Some Juglans regia wood comes from California, where 99 percent of the US walnut crop is grown, from trees too old to produce many nuts anymore.)
Fancier-grained walnut blanks can be very expensive, especially bolt-action blanks, because they require relatively straight-grained wood through the area holding the barreled action, with the weaker, less-stable fancy figure in the buttstock. This is why the butts of 2-piece stocks often have pretty nifty grain, even on factory rifles: It's much easier to find relatively short chunks of fancy walnut than the 3-foot sections suitable for bolt-action stocks.
Other kinds of hardwood are also occasionally used. Some varieties of maple are relatively lightweight, reasonably tough, and easily finished, but don't "work" as easily as walnut, partly because they often have really wavy grain, tending to splinter in odd directions. Birch and beech are sometimes used for stocks on lower-priced rifles, often stained darker to resemble American walnut, but rarely have any nice-looking figure.
However, any variety of hardwood supposedly has one glaring functional disadvantage for rifle stocks. Wood grows by sucking water out of the ground through the capillary action of its long, cylindrical cells, and also soaks up some moisture from the atmosphere. It never loses this tendency, even decades...