Twinning is common in gypsum crystals which often results in beautiful swallowtail habit.
I saw very nice exposure of swallowtail gypsum crystals in Cyprus. The crystals are several meters long and they all seem to be twinned.
These crystals formed in a shallow hypersaline lagoon in the Messinian (the last stage of the Miocene) about 6 million years ago. I have also written about laminated gypsum that is exposed nearby and formed roughly at the same time but probably in somewhat deeper water.
Twinned gypsum crystals (selenite) in Cyprus near Elediou. I know some of you might be worried about the hammer but I can assure you that I used it for scale only. I always carry it in my backpack when geologising and use it more often for scale (if pen is too small) than for smashing rocks. I would not consider destroying an exposure as beautiful as this and I very rarely attack outcrops at all. However, I do collect and often smash loose rocks.
good luck with that thesis!