Ethertallow is simply animal fat, usually beef or mutton, that has been exposed to a trickle of ethereal energy over the course of a week.
This is usually done by storing the tallow in a phial with contaminated water, which the fat soaks up slowly.
While the tallow goes in usually whitish in color, it usually emerges from the phial a dark purple, nearly black, with the consistency and malleability of wax. It is flammable, though not in the traditional sort of way. When exposed to flame or ignition, the fire goes from it's normal warm, orange-yellow color to a cool blue, or in some cases a near black blaze, that bathes the area in a subtle, clear light. It burns hovering impossibly over the tallow, seemingly floating above it about half an inch.
Ethertallow has a couple redeeming features that make it a good alternative to traditional light sources.
Gives off no heat
Of course, this makes Ethertallow a fairly useless bonfire starter. Ethertallow fire is, as you might have guessed, Ethereal in nature and thus traditional rules of physics do not apply. One can run their hand through Ethertallow fire with no worries of being burnt, though the threat of exposure to Ether is ever present. Of course, this makes those who delve into caves, mines and ruins much less susceptible to creatures who can sense the thermal image of a torch from miles away.
Again, you won't be making any signal fires with this, but in tight, enclosed corridors one need not worry about asphyxiating their friends, or occluding their visibility with a smoky Alitol-soaked (kerosene) or wood torch.
A little goes a long way
Wax is expensive, and it doesn't last long. Ethertallow, per pound, is the same price, though it burns for at least three times as long. And it's easy to shape into whatever figure it need be shaped into. With no pesky wick, the Ethertallow can be easily made both decorative and functional.
Keeps the beasties away
Most animals native to the material plane have evolved enough to sense the presence of Ether. Wild animals especially have honed into this sense, and take the presence of Ether as a dangerous threat. Even the small, trace amounts of Ether found in Ethertallow can be detected by most wild animals, causing them to stay on the periphery of those using Ethertallow torches.
OF COURSE you have to worry about the small amounts of Ether located in the tallow. As mentioned before, most animals native to the physical plane tend to shy away from Ethereal objects, which is why sorcerers and wizards often have a hard time keeping pets around. But there are those creatures, the abberations, that have been tainted by the Ether, and may be drawn to it's scent.
Finding a proprietor of Ethertallow is always tricky, as the process for forming it is usually left to Wizards and sorcerers, or Apothecarions with a penchant for dealing with hazardous chemicals or, in this case, immaterial extraplanar substances. And then of course, theres always the slight chance of overexposure to Ether, through the Ethertallow. Even a little, may potentially, be too much.