These cinnamon sticks are made from the inner bark of a South Asian species of evergreen called Cinnamomum cassia. These sticks, called quills, have an abundance of the organic compound cinnamaldehyde in their essential oils. This compound gives the spice its pungent flavor and distinctive warm aroma. Cassia's reddish-brown inner bark is thick and as it dries it turns inward from both sides curling to form a scroll. The quills are very rigid and difficult to break into piecesunlike Ceylon quills which have papery layers and crumble more easily.