This is a long grind and the achievement states 'Harvest any crafting materials 10000 times' for the max level of this to gain the Kresh Flower Red dye.
I am not far off now but I think the wording needs to be changed or redone as not all materials update the progress.
Specifically when you kill a creature and harvest the leather, you do not get granted progress to this achievement.
If you get credit for 'harvesting' runestones, ores and pulling water out of a skin (which is debatable if that actually is harvesting) then surely harvesting leathers from dead animals should too.