Natural fibers can be classified according to their origin into animal, vegetable or mineral.
Animal fibers may originate either from the glandular secretions of some insects as in case of silk, in which two fibroin strands are linked by sericin, or follicle bulbs of some animals as in case of wool, which has a molecular structure composed of keratin.
Vegetable fibers have elongated structures, mainly composed of cellulose and usually with round cross-section. They can be classified according to their origin into seed fibers, stem fibers, leaf fibers and fruit fibers.
As compared to the traditional man-made fibers, vegetable fibers present several advantages: abundance, low cost, low density, ability to absorb carbon dioxide from the environment, renewability and biodegradability.
In contrast, its main disadvantages are the high moisture absorption, low resistance to micro-organisms, low thermal stability and mechanical properties lower than those of man-made fibers.
Mineral fibers have their origin in the rocks with fibrous structure and are composed, essentially, of silicates. An example of a mineral fiber is asbestos.