Although Austin couldn't find a Magic Beast Crystal Core on the body of the Spider Leader, it doesn't mean that the body is useless. As a powerful Magic Beast of Silver Level, its shell is as hard as copper and iron. Even if a regular Beastman used a wooden spear with all their might here, it wouldn't do any harm. That's why Austin asked the Beastmen to step back.
If this Spider Leader's body were made into Shield Armor and its sharp limbs and tail tip attached to a wooden spear, the equipment produced would be no less than equipment made of copper and iron—perhaps even superior.
These materials from Magic Beasts are specifically acquired for making weapons in the Human World. However, Austin doesn't possess similar knowledge, so he can only make use of its durability attributes. If he could meet a Professional like an Enchanter, these materials could already be used to create Extraordinary Weapons.
