and welcome to NaturePlus.
This is one of those things that arises from where 'man-made' overlaps with 'natural'.
It is glass, or nearly glass. The conchoidal fractures show it is amorphous or cryptocrystalline, which indicates it is glass, or nearly glass. If the convolutions are just colour-based, I would say glass; if the convolutions are partly structural/crystalline, 'glass' would not pertain in the truest sense.
I think your specimen is man-made glass slag. Natural glass, such as obsidian, does not show such convolutions, at least not in my experience. The convolutions arise from impurities and flow when the glass was hot and fuild.
Have a read of the introduction on this page, then scroll down to the very attractive designs made from this material
(It is not quartzite, because that is distinctly crystalline, usually white, devoid of convolutions, and does not show conchoidal fractures.)