Most diamonds are found in Archaean rocks [0ver 2.5 billion years old]. There is no evidence of rocks of this age in western Afghanistan.
For more details of diamond-bearing rocks, consult: 'GEOLOGY – Vol. IV – Diamonds, Kimberlites, and Lamproites' by Paolo Nimis, and "Recent Advances in Understanding the Geology of Diamonds" in 'Gems & Gemology, Winter 2013, Vol. 49, No. 4.