This crystal doubles images, meaning that if you look through it that anything on the other side whether that be text, a person, or a photo of a volcano will appear to be doubled. But, why does this unusual optical property occur when utilizing a crystal of calcite? The answer is a fascinating phenomenon known as birefringence, which this video will explain through the expertise and analysis of a geologist.
And as a further aside, the image was made with an F-Droid app that is new to me but popped up 3 months ago apparently and has a lot of great features for stitching images and creating content: