Spider Wars, Chapter 1: I Really, Really Hate Clones - S5-E12
Continuity mistake: When the group of Spider-Men are observing the Green Goblin and the Kingpin's henchmen from inside the ventilation shaft, the outside edge of the shaft cover is first shown to be on the wall outside of the shaft, and then inside the shaft itself when the Armored Spider-Man pulls the cover off.
Answer: There are several substances in canon and non-canon that are resistant to lightsabers. Beskar, also known as Mandalorian iron or Mandalorian steel was used to make armor and weapons by the Mandalorian people. Cortosis was an ore that, when heavily refined, stopped lightsaber blades and blaster bolts. Phrik was another metal, used in Darth Sidious' lightsabers and the electrostaffs used by Grievous' robot guards. Neuranium was a very, very dense and heavy metal that was partially resistant to lightsabers, but was more often used to shield from scanners. The species orbalisk and vonduun crab had carapaces that could withstand the blow of a lightsaber.
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