So diamonds are an allotrope of carbon, that is formed when carbon is put under immense pressure, right? What other elements or compounds can you make into "diamonds", as in more durable, harder versions of themselves? What happens if salt or iron for example, is put under immense pressure, do they become harder too?
Interestingly, nobody addressed the reason why diamonds are hard in the first place. The pressure (and temperature) are not the reason why they're hard, only the reason why they are formed. The diamonds are hard because the carbon atoms are bonded together by sigma ($sp^3$) bonds, which are the strongest chemical bonds. Other materials exhibiting the same kind of bonding are potentially also very hard. But the small atomic number of carbon (Z=6) means that the bonds will be stronger than in higher atomic number atoms, such as silicon.
However, amorphous (non-crystalline, like glass) materials can also be extremely hard. There is even an example that is even stronger than diamond (although not naturally occurring). And this material is made of... carbon! Exactly like diamond. This material, dubbed Q-carbon, was discovered in 2015, and is even created at atmospheric pressure, and around 4000 K with the help of a laser pulse. And it is estimated to be 10-20 % harder than diamond.
Bottomline: gigantic pressures are not required to create extremely hard materials, and diamonds is no longer the strongest known material.
Here are some compounds that have other structures, followed by their hardest structure (based on Moh's Scale).
- Titanium dioxide: Rutile structure or Cotunnite structure
- Aluminum oxide: Corundum
- Silicon Oxide: Stishovite
- Boron Nitride: Wurtzite Boron Nitride
There are many, many more.
Don't know if this is exactly what you want, but Titanium metal has two allotropes ($\alpha$ and $\beta$). $\beta$ phase is stable at high temperatures, but you can quench $\beta$ phase titanium so that it exists (metastable) at ordinary temperatures. $\alpha$ phase is stronger and less ductile. Small amounts of alloying elements are often added to stabilize either $\alpha$ or $\beta$ titanium. (You get a similar polymorphism with zirconium metal and hafnium). $\alpha$ has a hexagonal crystal structure and $\beta$ is body-centered cubic.
Minor nit: it's not just the pressure, you also need the right temperature, and that's just scratching the surface. In general, most solids have multiple different structures with different properties, and adding other elements to the mix gives you even more possibilities. How and when exactly these structures form depends on a lot more than just pressure.
For example, pure iron can range all the way from tough and soft to hard and brittle. Pure iron grown in a single crystal (you'll need a vacuum or a reducing atmosphere for this, since iron is quite reactive) is exceedingly soft (softer than lead) and has a very low tensile strength (less than a human hair). Not very useful. As solid iron cools down, it changes crystal structure - each of these has different properties. The way (pure) iron metallurgy works is by exploiting the best structures and freezing them in place - for example, when you cool a piece of hot iron abruptly, it will form a dense and hard structure. The cooling is too fast to allow the whole piece to rearrange, so you get many grains of crystals placed one next to another. Add a bit of carbon, and you get the much stronger steel - the (slight) carbon impurity makes the whole thing tougher and stronger. Even smaller impurities of other elements (like nickel) add the whole thing some very interesting properties.
Sodium chloride (rock salt) doesn't really have any allotropes - it forms a very regular lattice with every sodium atom surrounded by six chloride atoms and vice versa. This is quite typical of giant ionic structures - the optimal structure is very stable. Just like diamond, salt is relatively hard and brittle.
The obvious comparison to diamonds are other gemstones. Most are compounds - like corundum (aluminium oxide) which forms rubies and sapphires, or beryl (beryllium, aluminium and silicon oxide) which forms aquamarines and emeralds. They tend to be hard, which is what makes them useful for jewellery - they resist scratching and keep a nice sheen. But gemstones are just the most obvious - the same minerals are present everywhere around you in the rocks and soils, and the only difference is the size and purity of the crystals. For example, most of sand is formed of silicon dioxide (silica, quartz) - it's basically the hardest thing that remains after all the softer minerals have been eroded (and formed various clays etc.). Silica itself has many allotropes, each forming under different conditions and with different properties. For your question, the most relevant is probably stishovite, a very hard form that forms under high pressures, but even less stable under standard conditions. It's quite similar to rutile, which is titanium dioxide. Silicon carbides are very interesting industrially nowadays - they're rather rare in nature (on Earth), but easily formed at high temperatures and stable at room temperature. Again, silicon carbide itself has many forms (about 250 - perhaps not too surprising given that it's formed from carbon and silicon, both very flexible elements). One especially relevant form is moissanite, which is used as a gemstone, and quite similar to diamond.
Some pure elements form multiple allotropes, though carbon is probably the most impressive element; there's a reason why carbon has its own branch of chemistry. Sulfur is pretty close to carbon, but not quite as useful for humans. You might have also heard about white phosphorous and red phosphorous - both are one of many allotropes of pure phosphorous, each with very different properties. Oxygen is an example of an allotropic element that isn't even solid - molecular oxygen and ozone coëxist with little trouble in both liquid and gaseous forms.
Among metalloids, the most notable is probably silicon - you see it everywhere around you in many of the minerals the Earth's crust is made of, but even pure silicon has interesting allotropes - amorphous (non-crystalline) silicon is used in solar panels, mono-crystalline silicon is used in integrated circuits, and it has forms analogous to carbon's diamond and graphene, with similar properties. Boron is quite interesting in having many different allotropes, including a super-conducting high-pressure phase.
About half the metals that occur naturally in significant quantities are allotropic at room temperature and pressure. The most notable is of course iron, and the flexibility allowed by this is one of the (many) reasons why iron is so important industrially. But I'll take tin for a closer inspection. Alpha-tin has a crystal structure like diamond. It is hard, brittle and non-metallic. Beta-tin is metallic, soft, malleable and ductile, and in cold conditions, tends to spontaneously transform to alpha-tin.
Some allotropes are stable at room temperature, others aren't. Diamond isn't stable - it spontaneously transforms to graphene, though very slowly (though consider that natural diamonds usually form over billions of years; in that scope, the transformation isn't slow at all). Beta-tin is stable close to 0 °C (up to about 13 °C, depending on the impurities), while alpha-tin is stable at room temperature.