First of all, at the time these substances were named, it was not known that they were both composed exclusively of carbon. They had very different properties, and basically had no more in common than steel and salt. Why one earth would anyone give them the same name? Second, even if it HAD been known that they were both made up exclusively of carbon, they are STILL two very different things. Diamonds are rare, graphite is common. Diamonds are expensive, graphite is cheap. Diamond is the hardest naturally-occurring substance known to man, graphite is so soft it's used as a lubricant. Diamonds are colorless (when pure), graphite is a dark grey. I could go on and on. The differences are nearly limitless. Why do ice, water, and steam all have different names? They are all composed, exclusively, of hydrogen hydroxide (HOH or H2O). They are far more identical to each other than diamond and graphite, because they have the exact same bond structure regardless of state (solid, liquid, or gas), while diamond and graphite have very different bond structures. Why do you and I have different names? We are both composed of the same elements, in the same proportions. For that matter, the proportions are pretty much the same among all animals, and probably for plants as well. So why isn't every human, cow, fox, fish, turtle, tree, flower, insect, and virus named simply "Bob"? Because there is a NEED to distinguish between species, and even within a species, between individual members. Likewise, there is a need to distinguish between diamond and graphite. If I came to you and told you I had a pound of "pure carbon" that I needed to sell, for $1,000, would you buy it? If it was diamond, that would be a hell of a deal for you. But if it was graphite, it's a very bad deal for you. Therefore, you would need to know which one it was before you agreed to the deal. And "carbon" doesn't tell you which one it is, does it? Regardless of how much two things might have in common, any time there is a NEED TO DISTINGUISH between them, they WILL be given different names. This is completely natural and reasonable. (I don't know why, but someone deleted the last 2-3 sentences of my answer. I didn't call anyone a moron or anything like that, so I don't know what could possibly have offended anyone. But, I will re-word the remainder of my original answer slightly and re-post it below, because this NEEDS TO BE SAID. It is a very important part of the answer to this question.) The way the question was asked assumes that the NATURAL and REASONABLE thing to do is to give two things the same name for the sole reason that they are, at the atomic level, composed of the same element. This notion is ABSOLUTELY PREPOSTEROUS. It is not at all natural or reasonable to name things in this way. In fact, that is just about the most UNnatural, UNreasonable thing I've ever heard of. At every level above the atomic level, and in every conceivable way, diamond and graphite are two very different substances. Even if they had everything in common except one property, that one difference would still be ample justification for giving them different names. In that vein, let's throw out the graphite for a minute, and let's talk about just diamonds. Now we have a lot more in common. Not only the same elemental composition, but the same molecular bonds, the same hardness, the same melting point, the same density, etc. And yet, even at this level, there are dozens of different names for just the CUT, GRADE, and COLOR of diamonds. But let's not stop there. There are literally hundreds of individual diamonds that all have UNIQUE NAMES. Just to name a few: The Hope Diamond, the Centenary Diamond, the Darya-Ye Noor Diamond, the Excelsior Diamond, the Heart of Eternity Diamond, the Oppenheimer Diamond, the Spirit of de Grisogono Diamond, the Tiffany Diamond, and the Uncle Sam Diamond. And here's a good one for you. The Great Star of Africa and the Lesser Star of Africa were both cut from the Cullinan Diamond, the largest rough gem-quality diamond ever found. Now, you can't get more identical than that. These three diamonds, as well as the 103 other smaller diamonds that were cut from the Cullinan, had everything in common except size and cut. And yet they STILL have different names. Because there is a NEED TO DISTINGUISH between them!