In the image displayed here, a crack that had formed when a crystal was broken slowly heals itself. Self-healing materials are nothing new but they belong to the polymer family. This crystal is made of dipyrazolethiuram disulfide and is more specifically a molecular crystal just like sucrose is. Now imagine you break apart a sugar cube and then press together the two resulting pieces. Will they ever reform to the original sugar cube? Not a chance. Yet somehow a dipyrazolethiuram disulfide crystal manages this as reported in Angewandte by Commins, Hara and Naumov (DOI). Their self-healing criterium: the re-attached pieces resist gravity and in quantitative terms the tensile strength is about 7% of the original.
You may have never have heard of dipyrazolethiuram disulfide. In internet searches the compound shows up only in relation to this very publication. Unfortunatly as to the how this compound was selected the article is silent. The compounds apparently is easy to synthesise; pyrazole meets with carbon disulfide and potassium hydroxide to form Pyr-C(=S)-S-S-C(S=)-Pyr. According to the authors the self-healing originates from homolytic cleavage of the disulfide bonds to form radicals. The radical groups can then shuffle and reconnect across the crack. This requires some mobility and a bonus SEM experiment nicely shows how an indent made on de surface of the crystal slowly dissapears over the course of several days.