There's a new fibre in town, and it's name is Qiviuk (pronounced key-vee-ook). Rarer than pashmina and more legal than shahtoosh (which isn't legal at all), Qiviuk is the inner down of the Canadian Arctic Muskox. I didn't find any numbers but the fibres are said to be half the diameter of merino (which would make it noticeably softer than cashmere) and eight times warmer.
The muskox is a ripe-smelling Pleistocene era contemporary of the wooly mammoth that's alive and thriving today. Their survival through the last ice age was largely due to a combination of their isolation in the far North, and their remarkable coat. They are shielded from the minus 50 degree C temperatures by a combination of an outer layer of guard hair that grows up to 24 inches long and an inner layer of down.
Because of their shaggy coats, muskoxen appear to be massive animals when in fact they are mere 400-800 pounders that are closely related to sheep and rarely grow more than chest high to a human. In May, when the arctic temperature begins to rise, muskoxen shed their inner coats, and the tundra becomes littered with the fleece. About 3,000 kilograms of Qiviuk are collected by the area's Inuvialuit people each season, which is not very much and one of the reasons Qiviuk is three to four times more expensive than cashmere.
Qiviuk garments are available directly from Qiviuk Boutique stores in Canada, and from resellers. Traditionally designed earth toned knitwear (scarves and sweaters) made from the stuff retails for between $350 and $6,000.