Alpaca grows on an animal of the same name. Alpacas are about the size of ponies and are related to camels and llamas (both of which also produce fiber that is suitable for knitting, though their fibers are not as readily available). Alpaca is shinier, smoother, and warmer than wool. It is also heavier and less elastic; as a result, alpaca garments often need to be reinforced with strong seams and firm edges to keep them from stretching out of shape.
Angora, which grows on the angora rabbit, is very soft, lightweight, fluffy, and warm. Because of angora’s fluffiness, a 100-percent angora garment will look as though it is surrounded by a cloud.
Cashmere is a very luxurious, soft, smooth, warm fiber produced by a strain of goat known as Capra hircus. Cashmere has much less elasticity than wool (for example, a cashmere ribbing will not pull in as much as a wool ribbing); instead, it drapes elegantly. Cashmere is less fluffy than angora but does “bloom” after it is knit and washed, that is, it releases a halo of fuzz.
Cotton grows on a bushlike plant. It is heavier and less elastic than wool, and for satisfactory results (to produce garments that do not stretch out of shape), often needs to be worked on smaller needles than wool. A lot of cotton is mercerized (treated with a caustic soda solution), which increases its luster, strength, and ability to absorb dye. Unmercerized cotton feels softer and warmer against the skin than mercerized cotton.
Linen is the product of the flax plant. It is strongly, slightly stiff, and lustrous. A linen garment will soften and lose some of its luster if it is machine-dryed, then left unironed.
Mohair grows on a kind of small goat called the angora goat. It is strong, lightweight, and warm, and when knit up becomes fuzzy. Mohair can range in texture from silky to scratchy, depending on the quality of the raw (unspun)fiber and how that fiber is spun and dyed. Kid and yearling mohair (from goats that are about 6 to 12 months and 18 months old, respectively) are generally softer than adult mohair. Because unspun mohair is slippery and delicate and can easily be damaged, it is usually machine-spun with wool-nylon core and binder yarns for durability; the core and binder yarns are very fine and can rarely be felt or seen.
Silk is the smoothest fiber of all. It is also strong and usually very shiny. It is made by a type of caterpillar called a silkworm. The silkworm produces silk in a cocoon. One cocoon, which can range from the size of a jelly bean to several inches long, is often made up of more than a mile a very fine silk. There are two main types of silk fiber used to make knitting yarns: bombyx mori and tussah. Bombyx mori silk (also known as mulberry silk) is produced by domesticated silkworms raised on a diet of mulberry leaves almost exclusively is softer, finer, and more lustrous than tussah silk, which comes from semidomesticated silkworms that consume a variety of leaves.
Wool from sheep is very popular for knitting because it is warm, springy, and strong. There are many breeds of sheep, each producing fiber with different characteristics (such as texture and fineness), though yarn labels rarely specify breed. Exceptions include merino wool, which is among the softest and smoothest wools available and Icelandic wool, which is lofty and soft.