How to prove that graphite and diamond are allotropic modifications of the same chemical element?

First, both graphite and diamond are written by the same formula: C. C is also the formula for carbon.

But these two substances have different ways and look different.

Graphite is usually black or dark gray in color, layered, brittle, brittle, easily destructible.

Diamond, on the other hand, looks very beautiful, it is a transparent, shiny, shining mineral. But it is much stronger than graphite, moreover, diamond is the hardest substance on the planet and can exist indefinitely.

Diamond and graphite are so different from each other because they have different crystal lattice structures. In graphite, it is atomic, and in diamond, it is cubic.