There are a number of materials used in Republic City for various purposes; buildings, vehicles, weaponry and armour. Since the early days of Republic City there has been much debate and research as to what makes for the most effective material, particularly in combat.
Characters who have access to multiple materials may make use of them together in weapons and armour, however, some specific methods are incompatible with certain combinations:
Smelting composite and metal/platinum
The most recent discovery was the creation of reinforced composite plastics, typically referred to as "composite" for short. Composite materials are unique in that they are not composed of any metal or earthen elements, and are completely synthetic in nature.
Composite is immune to bending by any benders and is the most lightweight material to use in construction. However, this makes it far less durable and strong, limiting its effectiveness in combat.
Composite objects can only be manufactured by individuals with the Smithing and Engineering ability.
The most commonly used material is metal, formerly known for its infamous ability to entrap benders, this advantage was drastically turned on its head after the discovery of metalbending. While it is difficult for earthbenders to manipulate it, due to its impure construction, some are able to bend metal to their command.
Metal is a middle ground material, having more strength and resilience at the cost of increased weight, making it harder to wield. It is also vulnerable to being manipulated by metalbenders.
Simple metal objects can be acquired and used by practically anyone, however, more complex metal items can only be made by individuals with Smithing and Engineering.
Platinum is a metal with somewhat unique abilities, unlike regular metals it is completely immune to metalbending while being considerably stronger and sharper. It takes considerable skill and effort to both forge and use platinum items due to it's nature, hence why only some nonbenders use it.
Platinum is considered the closest to the holy grail of materials, sharp, durable, immune to metalbending and even has the same weight as metal.
Any platinum object can only be made and used by individuals with Platinum Forging.