[bcg1]

Why don't they have stocks, diamonds, or any other types of valuables in their vaults?

[ThrustVectoring]

Uh, what? They do. The Chinese government owns approximately $60 billion worth of gold, and $1.2 trillion worth of US treasury bonds.

[bcg1]

Let me rephrase... why don't they have anything other than gold or cash equivalents?

[ThrustVectoring]

They're very much risk-adverse (for good reason). In exceptional circumstances, they will take on other assets, though. See - the US Treasury's stake in General Motors.