Being invloved in ceramic research (UK) I have studdied this area in great depth over the last eight years. When we talk of hardness I assume we are talking about flexural strength, In terms of the science of materials the minerals that make up a natural tooth are the (hardest) structure in the human body hence the reason we have periodontal ligements to stop us smashing our teeth when we masticate, It is very difficult to carry out a vickers hardness test on a natural tooth due to apperatus that are used to measure these figures, So quotes vary considerably anything from 400-900mpa, the average phelspatic porcelain is around 120-180mpa although some of the latest lithium disicate ceramics measure readings around 400mpa (But because these are synthetic porcelains particle formations are genreally encapsualted within quartz therefore giving a superior surface finish to the porcelain ie non abrassive) this figure aguably increases when enamel bonding an allceramic restoration as the bond to enamel is far greater than dentine bonding, This is due to the fact that when bonding to dentine you are reliant on mechanical adhession as oppose to chemical (mpa is the measurement for hardness
Mega pascals) I hope this gives a lttle more insight if anyone has any further questions on this topic I will be more than happy to discuss it further,
I once remember reading the research on a variety of porcelain that was based on hydroxy appetite the foundation mineral of a natural tooth it was apparently amaizing the only side affect was that restorations would suffer from tooth decay
Kind regards
Tony