Chitin

"An extremely thick composition of rare minerals and biomass, commonly found in a form of insect exoskeletons."

Chitin is a new material added with the Vanilla Factions Expanded - Insectoids module. It is primarily obtained from butchering insectoids, and can also be bought from traders or earned as quest rewards.

Chitin can be used as a material in structures, furniture, weapons, armor, apparel, etc.

Analysis
Chitin is twice as durable as steel, but 80% less durable than plasteel. It offers terrible blunt protection, but better sharp protection than steel. It is also ugly, having the worst beauty bonus when used to create sculptures. Despite having only 10% flammability, it offers little heat protection and only slightly more cold insulation than steel.

As a melee weapon, Chitin has 60% of cooldown time as steel usually do, resulting in 1.6 times faster attack than usual, the fastest in the game. Swifter weapons such as knife when made of chitin can result in terrifyingly fast attacks against poorly armored targets, dealing high damage in a short amount of time.

Chitin has 66% of the market value of plasteel, so unless building materials are scarce, the best course of action is to simply sell it for silver, or to create melee weapons with it in order to take advantage of the cooldown bonus.