Q: What crystal do you think was the most important ever and why?

Here are some possible answers without including the word 'ever'

Crystal structure of activated beta-2 adrenergic receptor in complex with Gs. Søren G. F. Rasmussen et al., Nature (2011), 477, 549–555. PDB entry 3SN6.

This year the most important crystal structure that has been solved is that of the ligand-free G-protein-coupled receptor (GPCR), which will help design more effective drugs. So this could be appointed as the 'most important' candidate for the year 2013.

Last year, the structures were established of several GPCRs bound to their ligands, including the structure of a GPCR bound to the G protein it activates in cells which was considered of sufficient significance to merit the Nobel Prize in Chemistry. Given the importance of the discovery, these crystals could be considered the most important crystals in 2012.

We had an incredible number of fundamental crystal structures studied and solved in the last 100 years of crystallography. Many of them had a tremendous impact at the time: Nobel prizes were awarded. But it is very difficult to find answers for the question above if we include the word 'ever'.

If the question is to be answered personally, the most important crystals ever were borazaro-phenanthrene crystals, whose phase transitions I observed in a polarizing microscope in the Crystallography Lab of the CNEA in Bs. Aires in the late 60s. They captured my imagination and helped me decide my future career as a crystallographer.

If the question is to be answered from the point of view of Science and Social Welfare, the most important ever are salt and sugar crystals: UNICEF reports that half of the children in developing countries that die of dehydration could be saved with a home-made recipe of diluted salt and sugar water solution: those items sometimes are not available in their homes.

A study by researchers from the School of Earth and Environment at the U. of Leeds and the Lab de Estudios Cristalográficos CSIC-U. of Granada found that gypsum starts off as tiny crystals of a mineral called bassanite. Most of us know this as Plaster of Paris as we use it in building, art-work, casts and fireproofing. Currently bassanite plaster is manufactured at a rate of 100 million tons per year by dehydrating quarried gypsum at 150°C. For the full article, visit www.leeds.ac.uk/news/article/3113/how_to_plaster_the_world_cheaply. (Photo credit: Javier Trueba/Madrid Scientific Films)

If we focus the answer on the wonders of Nature, the most important ever would be the giant crystals of selenite discovered in the caves of México, Chile, Perú and Spain.

And this is just in a general context: for us crystallographers, really obsessed people, the crystal we are studying is the most important ever. This is the spirit and drive that has led several generations of crystallographers to obtain remarkable achievements during the last century.