Cashmere is a fine hair which is combed from the underbelly of Capra Hircus goats.   These live in huge, wild, nomadic herds in China, Mongolia, Afghanistan and Iran.   They produce different types of fibre ranging from the finest and most expensive “white for white” Chinese, through coarser, darker Mongolian and then Afghan and Iranian which is suitable mainly for weaving.   Cashmere fibre is a rare natural harvest and a traded commodity so all fibre prices are subject to market fluctuations.
The finest cashmere hair is from China, the fineness a result of poor dietary grazing, continual herd movement and extremely harsh climate.   Fibre price is determined by several factors:
Average Fibre Thickness.
Grade A cashmere is 14 microns (14 millionths of a metre) thick but fibre can go up to 24 micron for Iranian weaving cashmere.
The finer it is the more expensive it is but the softer it feels.
Average Fibre Length.
Grade A cashmere is 34mm-36mm, the longer fibres giving higher tensile strength when knitting and better product durability.
Colour.
Very light “White for white” Chinese cashmere is the rarest, finest and most expensive cashmere fibre.   It is used mainly for white yarn and for pastel colours.    Not surprisingly, these light colours in yarn are more expensive to buy from a spinner.   Note that the more common brown cashmere cannot be used for light colours, only medium and darks.
Purity.
While Grade A cashmere is pure, some cashmere fibre can become unintentionally contaminated with other fibres through careless processing or can be intentionally adulterated with cheaper fibre to increase short-term margin or maintain low retail price points for High Street and Supermarket distribution.