“A simple answer to this question would be that the leaf of a plant which is used in cooking is referred to as a Herb and any other part of the plant that is used to flavor food is termed as a Spice.”
—Supreme Spice Blog: Difference between Spices and Herbs (via Slashfood)
So by this definition, tea is an herb and rooibos is a spice.
So by this definition, tea is an herb and rooibos is a spice.