Good silk is a natural animal fiber. Here's some info to explain why it is so expensive....
Silk is an animal fibre, produced by caterpillars belonging to the genus Bombyx. A single silk filament is the product of a series of stages derived from the cultivation of mulberry trees for feed to the propagation of the domesticated silkworm, Bombyx mori. During the caterpillar phase, the worm wraps itself in a liquid protein secreted by two large glands in its head. This secreted protein hardens upon exposure to the air. The resulting filament is bonded by second secretion, sericin, which forms a solid sheath or cocoon. Under natural conditions, a moth eventually breaks through the cocoon. In sericulture, the larva is killed in the cocoon by steam or hot air in the chrysalis stage before its metamorphosis. Sustained heat processing softens the hardened sericin so that the filament can be unwound.
The silk filament is a continuous thread of great strength measuring from 500-1 500 metres in length. Single filaments are too thin for utilization. For production purposes, several filaments are combined with a slight twist into one strand. This process is known as "silk reeling or filature".
Silk is a premium priced agricultural commodity, although its sheer volume is less than 1 percent of the market for natural textile fibres (Table 1). The international demand for high quality silk has multiplied. Appropriate cocoon-drying techniques and reeling operations are vital to supply good quality silk. Although new technologies are applied among major silk producing countries in Asia, many newcomers to the industry including non-traditional producers have not been transitioned to the methods that ensure high quality silk for export.