Steles look like long, slender twigs, though the length and style vary and may depend on the owner's preferences. There are many different models of steles, the appearance evolving depending on the times. Steles have a sort of aura to them, a ghostly imprint of their owner's personality.
Since steles are not designed to be used in direct battle against demons, steles do not harm them, though demons tend to recoil when exposed to a stele due to the material it is made of—the heavenly metal adamas.
Only Shadowhunters can use steles to create Marks; steles will not even produce a line when used by mundanes or Downworlders, but it will cause no harm to them.