This is a large section of indigo gabbro that has been polished into the shape of a 6 sided obelisk. The base of the obelisk is flat, allowing for aesthetic presentation.
Indigo gabbro, also known as "Merlinite", is a mineral conglomerate that occurs and is mined in Madagascar. The typical colors of these basalt rocks are a deep indigo-purple and black. It's frequently polished into decorative items and table tops.
The word "gabbro" refers to a large group of dark, coarse-grained, intrusive igneous rocks that are chemically equivalent to basalt. The rocks can contain a mixture of feldspar, chlorite, serpentine, muscovite, pyroxene, hercynite and magnetite within in a charcoal colored matrix.