One of the things that's bound to come up eventually is interaction with a culture that doesn't have metalworking skills, whether it be a culture that never developed it or one that has been isolated and no longer has access to metal.
Some weapons are unaffected by a lack of access to metal - the bola, club, net, sap, staff, and whip can all be made without metal with no alteration of their statistics.
All other weapons are assumed to incorporate metal into their striking surfaces (including such things as arrowheads and lead sling bullets). Alternate available materials are bone, stone, and wood.
Bone: While called bone, these can be made from many different kinds of hard animal remains. Historical examples include darts of caribou bone and spear points of walrus ivory (Inuit), a sword made of coconut husk and wood with shark's teeth embedded as a cutting surface (Kiribati), and daggers made from stingray barbs (Maori).
Stone: In addition to stone age axes, picks, and throwing stones, this would also cover things like using smoothed stones in a sling or the Mexica macquahuitl, which used obsidian blades glued to a wooden paddle to create a sword.
Wood: This covers weapons that are made just from hardwood, with points or edges charred and sharpened. Very low-tech spears and arrows were made this way, before points of other materials were available.
Rules: A bone or stone weapon reduces the die size of the weapon by one, so a weapons that does d4 now does d3, d6 becomes d4, d8 becomes d6, and d10 becomes d8. A wood weapon reduces die size by two.
A bone weapon counts as half its normal encumbrance, and costs half as much as normal. A stone weapon counts as 3/4 its normal encumbrance, and costs 30% as much as normal. A wood weapon counts as half its normal encumbrance, and costs 10% as much as normal.
Exotic materials may alter these qualities - for example, dragonbone may deal normal damage, while having the same weight as bone (but being far more expensive!), while ironwood might only reduce damage by one die size, count as normal encumbrance, and cost 20% of normal.
Example weapons: A bone two-handed sword will count as half a stone for encumbrance and deal 1d8 damage, costing 7.5 gp to acquire. If it was made of wood, it would be the same weight, deal 1d6 damage, and cost only 1.5 gp. A completely wooden spear would cost just 3 sp, and deal 1d3/1d4 damage, while a wooden dagger would also cost 3 sp and deal a measly 1d2 damage. A set of 20 stone-tipped arrows would cost 3 sp, count as 3/4 of an item, and deal 1d4 damage at their bow's normal range.