 Fibre diameter is the most important characteristic of wool in determining its value.Every fleece comprises a very wide range of fibre diameters—for example a typical Merino fleece will contain fibres of as low as 10 microns in diameter, and there could be fibres with diameters exceeding 25 microns, depending on the age and health (or nutrition) of the sheep.What is usually referred to as wool's "micron" is the mean of the fibre diameters or average diameter.