#thorianite

It has a hardness of 6.5 but is considered brittle, is highly radioactive, contains significantly more thorium in it than most other radioactive substances to the point where you shouldn’t even have exposed skin or breathe openly in close proximity for extended periods, is named after Thor, is either black, reddish-black, brownish-black, or very very dark grey, and forms in cubic crystals.

From here, we can figure this:

Personality: your options seem to border on very toxic behavior, or serious misunderstanding and potential. Black is a color of death, uncertainty, and stability; as a radioactive mineral Thorianite could be extremely violent, unapologetically shattering other gems in battle and quietly letting organic creatures grow sick in their presence; those who are let live are subject to a pervasive attitude, as they’re probably not very friendly. If Thorianite were to take a positive route, they could be literally radiating potential, uncertain, afraid, but eager to do something. They might have a strong urge to learn, to find self-worth, a purpose, and in doing so could accomplish great things, wanting to try all things. This can also lead to either shyness or loneliness, as black is a color of hiding/subtlety, and not many things can stay close to a radioactive mineral for long.

Weapon: This gem has a lot of potential as a homeworld gem, focusing their radioactive properties into a laser or projectile weapon. If not, explosive projectiles such as molotovs or any weapon that could provide a blast radius is possibly ideal for them. You can also go with their name and give them a hammer.