I don't found the Mohs hardness of plutonium but:

- the Brinell hardness is 242 at r.m.

- the Vickers hardness is 255 at r.m.

After a hardness conversion chart an equivalent Mohs hardness may be 3,7.

The hardness of rhenium is 7.0 on the Mohs scale. It has a hardness of 2450 MPa on the Vickers scale and 1320 MPa on the Brinell scale.

I don't found the Mohs hardness of technetium; probable was not measured. The Brinell hardness is 112 and the Vickers hardness is 151.

In the common Mohs hardness scale used in geology, 10 is the maximum number, the hardness of diamond.

There are several other hardness scales such as Vickers, and Brinell; but these have specialist engineering (not geologic) applications.

In brief, Brinell 28 would be rather soft - about that of copper. In Vickers, this number would represent something even softer - around that of gold.

Aluminum oxide, as corundum, defines 9.0 on the Mohs hardness scale.