This is a great sample of leaf-thin white transparent calcite from Malmberget.
The speciment is 150 x 120 x 27 mm, and 478 grams.
I have seen similar (smaller ones) go for wild prices, so seach the net and see yourself. Please also watch the video for a better view, the video can also be seen on https://youtu.be/CyR6Yn9d4Cs
It is a bit hard to catch the transparency of the crystals, so I have used some extra light when taking the photos.