Pliny is especially observant on crystal habit and mineral hardness for example.

The Mohs scale of mineral hardness determines which stones or other objects will scratch what surfaces.

Its hardness is 6 on Mohs scale of mineral hardness.

About ten years later, Friedrich made a scale from one to 10 to describe mineral hardness.

On the Mohs scale of mineral hardness, quartz rates no higher than a 7, less for types with impurities.

The coral measures 3.5 on the Mohs scale of mineral hardness so besides being easy to work, it is also easy to damage.

Dentin rates approximately 3 on the Mohs scale of mineral hardness.

Quartz has a mineral hardness of 8.

On mohs scale of mineral hardness, diamonds are scored as 10 (the highest score possible).

Knorringite is a greenish blue color with a Mohs scale of mineral hardness of six to seven.