The standard 'Scientific
' explanation is that the carbon-carbon bond
s in diamond are too stable, no enzyme
would be able to overcome the energy barrier
necessary to disassemble diamond.
However, diamond is something of a special case for carbon compounds (oh, fullerenes are probably pretty inedible). There are organisms that 'eat' rocks, reduce gold salts to elemental gold and other improbable diets (from the point of veiw of sugar-eaters). There are bacteria that derive energy from sodium and some that produce hydrogen or eat methane. Many organisms can synthesise silica polymers - and I don't in principle see why you couldn't engineer bacteria to make silicon chips.
The message is that it's more surprising what Life can do than what it can't.