Silk in general is a natural protein fiber. The shimmering appearance for which it is prized comes from the fibers' triangular prism-like structure, which allows silk cloth to refract incoming light at different angles.

There are several types of silk, according to the insect that produces it. The silkworm silk is composed of amino acids dominated by alanine, serine, and glycine; the spider silk has the same amino acids but also has a substantial quantities of other amino acids, such as glutamine, tyrosine, leucine, and valine. For instance, the silk fibroin produced by a silkworm belonging to the family Lasiocampidae (order Lepidoptera, class Insecta) contains a large amount of glycine and alanine, as well as hydroxyl, acidic, and basic amino acids (Source: www3.interscience.wiley.com/cgi-bin/abstract/104032369/ABSTRACT.)

The insects produce silk by a mechanical mechanism, and a physiological process. In the case of the spiders, they start spinning and synthesizing the silk in specialized silk glands in the abdomen. The orb-web-spinning spiders can produce silk from six different types of glands. Each of the six fibers has different mechanical properties.