Toughness (mining)

Toughness is a passive trait exclusive to miners. It increases your Stamina, with patch 3.0.8, scaling with mining skill level.

This is roughly the same bonus to stamina as that given by the crafting professions, making mining very attractive to tanks, particularly those that benefit from extra stamina, such as Blood Death Knights and Guardian Druids. The total stamina can be increased additionally with talents.

Rank table
The Stamina does not appear as an addition (i.e. 100 + 3,) rather it seems to directly increase your Base Stamina (100 becomes 103.)