A six-sided mineral crystal that is very hard is most likely quartz.

Here's why:

* Six-sided: Quartz crystals often form in hexagonal prisms, which have six sides.

* Very hard: Quartz is known for its hardness, ranking 7 on the Mohs Hardness Scale (diamond is the hardest at 10).

While other minerals can form six-sided crystals and be hard, quartz is a common and well-known example.