What are spade bits made of?
Spade bits are almost always made from tool steel, which is a type of steel with a high carbon content that is designed to be hard and strong.
Tool steel is not as hard as high speed steel (HSS), which is used for some other drill bits. However, it is cheaper, and is more than hard enough to bore through wood (including chipboard and fibreboard), plastic-laminated wood, acrylic and plasterboard.
For more information on types of steel, see: What are drill bits made of?
You may find high speed steel spade bits made by some manufacturers. These are more difficult to sharpen (you will need a diamond file or bench grinder), but they will stay sharp for longer.