It has the same shape as the leaf symbol in German-suited playing cards but its appearance is more akin to that of an upside down black heart with a stalk at its base.
It corresponds to the suit of leaves (Laub, Grün, Schippen or, in Bavaria, Gras) in the German-suited playing cards.
The spade symbol is a very stylized spearhead shape, pointing upwards, the bottom widening into two arcs of a circle and sweeping towards the centre to then form a sort of foot.
In the official skat tournament deck, spades are green, assuming the color of their German-deck equivalent.
Not shown is the Knight of Spades used in Tarot card games: Four-color decks are sometimes used in tournaments or online.