Asbestos

Lovely ceiling tiles at Cleeve Abbey with asbestos in them

Pliny the Elder has a chapter in his Natural History on something he calls "live" linen:

It is generally known as “live” linen, and I have seen, before now, napkins that were made of it thrown into a blazing fire, in the room where the guests were at table, and after the stains were burnt out, come forth from the flames whiter and cleaner than they could possibly have been rendered by the aid of water. It is from this material that the corpse-cloths of monarchs are made, to ensure the separation of the ashes of the body from those of the pile. [Book XIX, Chap.4]

The Greeks, we are told, call this material asbestinon* ["inextinguishable"]. It was used to wrap royalty for their funeral pyres, because their ashes could be kept separate from the wood ashes. Charlemagne also used to entertain his guests by throwing the tablecloth into the fire after the meal, then removing it with all the stains burned off. Marco Polo was also shown cloth that whitened in fire and did not burn.

Asbestos was not just for cute tricks. It was used as insulation for armor, and in 9th and 10th century Afghanistan it was used to eat off. It also promoted belief in miracles:

the most fascinating use of asbestos during the period was as a magical cross sold by traveling merchants.  The crosses, cut from asbestos, looked like very old, worn wood and were advertised by merchants as "true crosses" made directly from the wood of the cross upon which Jesus Christ of Nazareth died.  To illustrate the magical cross's powers, the merchants would throw the wood into a fire where it would remain undamaged. [link]

The health risks of asbestos did not escape notice, however. Pliny did note that slaves who worked weaving asbestos had lung problems. Romans—who called it  amiantus, "unpolluted" (because of its ability to come clean from a fire)—understood that buying slaves who worked with asbestos was a bad return on investment, because they died younger than other slaves.

*"Asbestos" is used to describe six silicate minerals with thin fibrous crystals that can be woven into thread and thence into cloth.