If you get both the diamond spade and pickaxe in your hotbar when you click on the spade and it is 'in your hand' you see it as a pickaxe. To show this simply click between a diamond spade and then a diamond pick in your hotbar, and the item in your hand remains looking like a diamond pick.

This doesnt apply to the mining qualities of the spade - it still mines gravel quicker than a pick (as normal) it just looks the same.