Best viewed large to see how these crystals formed. The crystals grow by expanding when the mineral is heated and then cooled. It is called a "hopper mineral' for the structural pattern of crystals.
Third of 5 images in a row of the mineral bismuth, a metal that has some radioactivity and is often laboratory-grown to produce such large crystals. This specimen is about 2" across. In different light it will catch rainbow colors. See the links below for fascinating information about bismuth and how it can be grown.
Seriously, you could make these at home!