Clearing the Chromium Tanning Confusion
There is much inaccurate information in circulation regarding the use of chromium salts and their associated safety for the tanning of leather.

Approximately 80% of global leather production is tanned with safe chromium 3 salts. It is a highly effective tanning agent producing a leather with a flexible range of properties making it suitable for many end uses.

Chromium is a transition metal that can exist in a number of different oxidation states each with distinctive properties. 

Metallic chromium
This is a steel grey, hard metal found as chromite ore (it does not occur in the metallic state naturally).  It is used to harden steel, to manufacture stainless steel, and to form alloys. It is also used in plating to produce a hard corrosion resistant surface.