What if pencil lead were diamond?

Draw a line with a pencil. The gray part leaves a mark.

That gray part is made of carbon. Diamonds are made of carbon too! What if a pencil had a diamond inside?

Pick up a pencil and scribble. The dark core that leaves the mark is called lead, but it has no lead metal in it at all. It is graphite, a soft, greasy form of carbon.

Here is the surprise: a diamond is also pure carbon. So what if the core of your pencil were diamond instead of graphite? This is a thought experiment. We can't easily build a diamond pencil, but we can follow the clues about how the two materials behave.

A pencil core is mostly graphite mixed with clay, a recipe developed by the French engineer Nicolas-Jacques Conté. Graphite was long mistaken for the metal lead, and the name stuck even after Carl Wilhelm Scheele showed in 1779 that it is a form of carbon.

Diamond is also elemental carbon. Two materials made of the same atoms could hardly behave more differently. This lab asks what would change if a pencil's core were diamond, and uses the answer to explore how atomic structure sets a material's properties.

When you write, the paper rubs the pencil tip.

Tiny bits of gray graphite come off. They stick to the paper and make your line.

Paper is rough. As you drag a pencil across it, the paper's fibers scrape tiny particles of graphite off the tip, and those particles stay on the page as your line.

Pencil makers mix graphite with clay to change how soft the core is. A softer core leaves more particles and a darker line; a harder one leaves fewer and a lighter line. Common writing pencils are numbered from 1, the softest, to 4, the hardest. The everyday number 2 is medium.

Britannica describes a pencil core's hardness as how much it resists abrasion by the paper's fibers, and a mark's darkness as set only by the size and number of graphite particles deposited. Pencil makers tune this by changing the proportion of clay to graphite.

Artists' pencils run from soft 8B to F, and technical pencils from HB to 10H. So writing depends on the core being soft enough to wear away, a property that comes from graphite's structure.

Now pretend the pencil has a diamond inside.

Diamond is super hard. The paper might not rub any bits off. Your line might be very faint, or not there at all!

Imagine the same pencil with a core of diamond. Diamond is the hardest natural substance known, far harder than paper. The paper's fibers might scrape off almost nothing, so you might see hardly any line at all.

A diamond is also often clear, not dark gray, so any tiny bits that did rub off might not look gray like graphite. Instead of writing, a sharp diamond point might scratch or cut the paper. These are ideas, worked out from what we know about the two materials.

If hardness is resistance to abrasion by paper fibers, a diamond core, at Mohs 10 against graphite's 1.5, would barely abrade, so very few particles would be deposited and little mark would form. Gem diamond can also be colorless and transparent, unlike opaque, dark graphite.

These consequences are reasoned, not tested: no one makes diamond pencil cores, and real outcomes would depend on the diamond's shape and the paper. What is certain is the contrast in the materials themselves.

Graphite carbon is stacked in flat sheets. The sheets slide apart easily.

In diamond, each carbon holds on tight to four others. That makes it very hard.

In graphite, carbon atoms join into flat sheets made of six-sided rings, like chicken wire. The sheets are stacked with wide gaps between them and are not held together tightly, so they slide and flake apart. That's why graphite feels greasy and is soft.

In diamond, every carbon atom is joined to four neighbors, all at the same distance, all through the crystal. This dense, strongly bonded structure has no loosely held layers, and diamond is extremely hard.

Graphite crystallizes in the hexagonal system: rings of six carbon atoms form widely spaced sheets, and the layers are less tightly bound to each other than the atoms within each layer. The layers part easily, giving graphite a Mohs hardness of 1.5, a greasy feel and good conductivity of heat and electricity.

In diamond, X-ray diffraction shows each carbon atom linked to four equidistant neighbors throughout the crystal. Britannica notes that this dense, strongly bonded structure is why diamond's properties differ so greatly from graphite's, even though pairs of forms of one element are usually rather similar.

Graphite is perfect for pencils because it is soft.

Diamond is perfect for cutting because it is so hard. Workers put diamonds on saws and drills.

Each form of carbon is used for what it does best. Graphite goes into pencils and lubricants. It conducts electricity well and is used in electric-motor parts and in batteries.

Diamond's hardness puts it to work in industry. Small or flawed diamonds, too poor to be gems, are mounted on drills, saws for cutting rock, glass cutters and grinding wheels. Diamond for tools can be mined, or made in factories, usually by squeezing and heating graphite.

Britannica lists graphite in pencils, lubricants, crucibles, polishes, electric-motor brushes, steelmaking electrodes and, increasingly, the anodes of lithium-ion batteries. Industrial diamond, too flawed or small for gems, serves as a cutting tool or abrasive: rock-coring drills, saws for rock, glass cutters, wire-drawing dies and grinding wheels.

Synthetic diamond is usually made by subjecting graphite to very high temperature and pressure; General Electric did so in 1955 at pressures approaching 7 gigapascals and above 1,700 °C with a metal catalyst. Because synthesis is expensive, most synthetic diamond is grit or small crystals for tools rather than large gems.