Diamond is made of carbon which is during maybe 3 billion years under pressure is formed in the earth. Imagine you found a big source of diamonds such as coal...how much is the energy of the diamond compared to the coal, and how should you get the energy out of it?
Diamond is just one of the allotropes of carbon. There is nothing unique about diamond that allows it to store significantly more energy than any other compound. It is true, however, that much energy is required to separate carbon atoms in a diamond lattice. This is due to the high bond dissociation energy of a carbon-carbon covalent bond.
Although bond dissociation energy in a C—C bond is relatively high, it is dwarfed by the energy predicted to be released by the mass-energy equivalence theorem.
$E = mc^2$
The nuclear mass of 1 mole diamond is about 12 grams. So the theoretical energy released in a fusion reaction involving one mole of diamond is around 1*$10^9$GJ.
I don't think this is the answer you were expecting. But this is the amount of energy in 1 mole of carbon. The method of releasing all this energy is a different story. Nuclear fusion would release a significant portion of this energy but not all of it.
The trouble with the question is that is misses an important aspect of chemical energy which is the question: relative to what?
The C-C bonds in diamond are strong but this means they need a lot of energy input to break them up not that they "contain" a lot of energy. If we form diamond from gaseous carbon atoms then we release a lot of energy (though this is a theoretical idea since it isn't a practical way to make diamond).
Diamond stores slightly more energy than graphite (~2kJ/mol at normal conditions). This means that if we could convert diamond to graphite we could release a really small amount of energy. This isn't practical or desirable.
Both forms of carbon can release energy when they burn. So, in a sense, they store energy relative to carbon dioxide. This is because carbon-carbon bonds are higher in energy than carbon-oxygen double bonds so converting one to the other releases energy (this is why coal is a good energy store and fuel).
So when you talk about releasing energy from chemical, you need to specify the reaction you have in mind. Absolute energies are pretty meaningless.