“…What’s the magician’s traits? Things like clothes, weapons, the way of speaking, or the way they act.”

Kamijou then mentioned everything he knew about "Unabara Mitsuki" to Index, including how he had changed into somebody else’s appearance and the stone knife he wielded.

After three seconds of short silence, Index replied, “The black stone knife is obsidian, right? A spear made by using a mirror to reflect starlight… that should be the Spear of Tlahuizcalpantecuhtli.”

[...]

“Simply put, humans can only use a replica of the Spear. If the real Spear was used, all humans would die. I think the theory behind the replica should be using the obsidian knife as a mirror? The light of Venus from the sky gets reflected by the mirror and reaches the enemy as the Spear. On the other hand, if you don’t get hit by the light, then you can avoid the attack by the Spear. If properly handled, Touma’s right hand can block the attack, too; the main point is how to find the unseen path of attack."



Toaru Majutsu no Index vol.5 ch.2 pt.15