There are hundreds of materials in common use. To name some of the most obvious:
Why are there so many? Why don't we have a universal material now?
Fifty years ago, some people thought plastic would replace everything else. Plastic is cheap and comes in a myriad of forms. It seemed to have the potential to become the universal material. It has in fact become ubiquitous, but it hasn't superseded other materials. All the materials that were in common use before the advent of plastics are still in common use.
Each material has its place. New materials replace older ones for some purposes -- try to find a wooden tennis racquet in a sporting goods store today -- but usually new materials find new niches, and the old ones remain. It's hard to think of any material that has been completely superseded. Even pottery, which was probably the first synthetic material, is still in use. You can go to a garden supply store and buy a plastic flower pot, or you can buy a flower pot that is basically the same as the pots our ancestors used thousands of years ago. Bronze, the first alloy, was once common and is now rare. It has been replaced for most purposes by newer metals. Nevertheless some things are still made of bronze. Whalebone, ivory, and hemp are no longer used, at least not in America, but this is for political reasons, not because they have been superseded as materials. Maybe papyrus has fallen entirely into disuse. I don't know.
We are supposed to believe that diamondoid materials are going to become universal, and everything else will be obsolete. We are entering "The Diamond Age" (the title of a science fiction book). This is nonsense. Whether diamondoid materials become as ubiquitous as plastics depends on what properties they have and what they cost. Even if they can be made with a vast range of properties, like plastics, and even if some of them are "as cheap as potatoes" (as Ralph Merkle says), they still won't replace all other materials for all purposes. They will replace some materials for some purposes, and they will find new niches for themselves, just like any other new material. They will be an addition to our repertoire of materials, not a replacement for all the others.
Putting it more formally: to show that diamond will become the universal material, you would have to prove that the set of diamondoid materials is dense in the set of all materials, and that the cost of making a diamondoid material with certain properties is always less than the cost of making any other material with equivalent properties.
I am using the word "dense" in a technical sense which may not be familiar to everyone. Given two sets A and B, A is dense in B if every point of B is a limit point of A. For example, given a real number x, you can approximate x as closely as you want with rational numbers, but you can't approximate x as closely as you want with integers. Therefore every real number x is a limit point of the set of rationals, but not a limit point of the set of integers, and therefore the rational numbers are dense in the real numbers, but the integers are not dense in the real numbers. These concepts are explained in most books about elementary analysis or topology.
In the present case, what needs to be proved is that any material can be approximated by a diamondoid material, to such a close approximation that the difference is negligible for all practical purposes.
It may indeed be true that the set of diamondoid materials is dense in the set of all materials. This hasn't been demonstrated, but it could be true. Maybe subtle variations in the arrangements of carbon atoms can simulate cedar, silicone, gold, gallium arsenide, nitinol, and every other material. That's quite a stretch, and the burden of proof is on Eric Drexler. The space of materials spanned by all possible arrangements of carbon atoms is much smaller than the space spanned by all possible arrangements of all elements. On the other hand, the set of rational numbers is much smaller than the set of real numbers (one set is countable and the other isn't), but that doesn't stop us from using rational numbers to approximate real numbers. So the answer is not obvious.
Of course it isn't really necessary for diamond to replace all other materials. In Nanosystems, Dr. Drexler has demonstrated that diamond could replace some materials for some purposes, if the price is right. The big question, then, is how much it will cost. If it becomes possible to use "atomic positioners" to put atoms in place one by one, will that be cheaper than assembling atoms with bulk processes? This question will be discussed on another page.
It should be noted that, in the list of materials at the top of this page, quite a few (i.e. the ones derived from animals and plants) are already made atom-by-atom. However, there are two ways to do this. Atoms can be guided into place by cellular machinery (e.g. ribosomes), or they can (supposedly) be picked up by "atomic positioners" and put into place. The latter method is what Eric Drexler calls "positional synthesis," and that is what we are concerned with here.
If something can be made with chemistry,
it will cost more to make it by positional synthesis
Note added August 4, 2004: I'm redesigning the site and converting everything to CSS. This is one of the pages I'm experimenting with (since it needed to be rewritten anyway). Don't be alarmed if it appears that you have suddenly arrived at a different site! I will make all the pages consistent as soon as possible. However, next week I will be at SIGGRAPH, so I won't be working on the site.