Trade Goods come in many forms. In many places, gold and silver are not common and people will often trade raw or refined goods for other goods or services. Trade between kingdoms can also be disrupted when there is no standard between kingdoms with their coinage. In most campaigns, the GM assumes coins are standardized. This is seldom the case in reality and ambitious DMs will take this into account in complex campaign settings.
Raw Materials are those which are collected "as is" and sold to be combined or refined as someone needs. These are often the easiest to collect and are amongst the most common but cheapest of Trade Goods.
Much more difficult to gather, gems and jewels are still often found and traded as a more portable wealth than gold. Often people will find raw gems or mine raw jewels to sell. These raw, uncut materials only go for 1/10th the price of cut and polished stones.
Some materials are often processed from their raw form for ease of transfer. Refining the material generally increased the cost of the item greatly. Since the refined form is easier to carry and trade, it is sought after above the raw form in many cases.
Raw spices are a common trade staple for traveling merchants. Adventurers can use many of these spices and herbs during their travels. Often, spices become a method of payment like other goods when gold or other wealth is unavailable.
As difficult as it can be to think of it, domestic animals are often used as trade goods. A farmer may not be able to pay a group of adventurers for the help he has given them so he will give them an animal or two so he isn't indebted to them. Some peddlers travel around buying and selling these animals instead of other trade goods.