You've touched upon a really interesting material. Carbon is a material with many allotropes (i.e. different structural forms of the same material). Other forms of carbon you may be familiar with are graphite and carbon nanotubes. Carbon has four valence electrons, which means it tends to form sp3 and sp2 hybridized bonds. These bonds are covalent bonds, which are very strong. So why isn't coal as hard as diamond?

It has to do with bonding.

What kinds of structures are possible in carbon allotropes?
Coal is indeed composed of mostly carbon, but there are many other substances also present. In fact, coal is a huge hydrocarbon, shown

You can see there are benzene rings and linear chains of carbon spewed among many functional groups.

Contrast that with diamond, which is exclusively composed of C-C covalent bonding in a periodic tetragonal fashion.

click here for image

As you can tell, the packing of this hydrocarbon isn't as good as it is in diamond. It takes less energy to disrupt the structure in coal than it does in carbon because it is easier to break secondary bonding that occurs within the hydrocarbon itself. These include Van de Waals forces and hydrogen bonding. There are also just more covalent bonds to break, which adds to the hardness of diamond.

But having many types of bonding can also be advantageous. If you contrast graphite and diamond (above), even though both are essentially C-C bonds, their mechanical properties are very different. Graphite is composed of sheets of carbon that are held loosely by weak Van de Waals forces. The weak interaction between layers is what makes graphite ideal in pencils for writing and as a lubricant- when you're using graphite, you're just pulling apart all these layers!

The structure of graphite is also easily translated into carbon nanotubes; you can think of carbon nanotubes as rolled up sheets of graphene (a single sheet in graphite). Carbon nanotubes are an exciting area of active research. Various forms of carbon nanotubes have been found to be harder than diamonds, more conductive than copper, and stronger than steel!

Hope this helps!
Best,

Thank you for your question! Carbon, as with many elements, can arrange its atoms into several different geometries, or "allotropes." In pure diamond, every carbon atom is covalently bonded to exactly 4 other carbon atoms in a very specific and energetically favorable geometry. The diamond cannot be broken or scratched unless many covalent bonds are broken, which is difficult to do. In another common allotrope, graphite, every carbon atom is covalently bonded to only 3 other carbon atoms, and the atoms are arranged in sheets that are not covalently bonded to each other. The sheets can be broken apart easily, ultimately meaning that graphite can be easily scratched. Coal is composed of particles of different allotropes of carbon, and some "amorphous carbon," which has no defined geometry in its atomic structure. Without a continuous network of covalent bonds, coal is easily scratched (i.e. it is not hard).

First, you need to understand that there are many different types of coal – lignite, bituminous, anthracite, graphite, etc. – that vary in the amount of carbon present (some types have other elements like hydrogen and sulfur). For the purpose of this question, "coal" is referring to "graphite", which is made up almost entirely of carbon atoms (like diamond).

Looking at a dirty piece of coal and a beautiful diamond, it's hard to believe they are composed of the same main element – carbon. Coal and diamond are two examples of carbon allotropes, where the carbon atoms are bonded together in different configurations. These structural differences result in very different material properties, such as hardness. According to the Mohs Hardness Scale, numbered 1 (softest) to 10 (hardest), coal is a 1 or 2 whereas diamond is a 10!

The carbon atoms in coal are arranged in 2D sheets, where each carbon atom is bonded to 3 other carbons to form hexagonal rings. These sheets are held together by weak bonding forces called van der Waals forces, which is why coal is so soft (think: graphite in your pencil). On the other hand, each carbon atom in diamond is bonded to 4 other carbons, forming a rigid, 3D crystalline lattice. These strong covalent bonds between the carbon atoms give diamond its superior hardness.

They have different chemical structure. In diamond, each carbon atom is covalently bonded to another carbon atom in a tetrahedral arrangement, making a uniform, tight lattice in space. Coal actually isn't a mineral (and isn't pure carbon), but graphite is. Graphite however consists of sheets of carbon atoms with a floating double bond between the different atoms, and the sheets themselves are bound quite loosely. Thus any kind of pressure will cause the sheets to fry apart, which is why graphite, unlike diamond, is so soft.

The reason diamond is so hard has to do mainly with its crystal structure, which describes how the atoms pack. Diamond is made of carbon atoms that are packed in a very specific fashion. The carbon atoms are all closely interacting with each other, and there are strong chemical bonds (called covalent bonds) between the carbons. These covalent bonds create an infinite 3-dimensional network that is very rigid, and this is what gives diamond its special properties.