Petal brushes have been designed for making petal shapes. There are two kinds:
One is the Flora brushes which have a flat ferrule and bristles tapered to a fine point like a leaf. Designed particularly for painting petals but fantastic for many other things.
The other type of brush known as a petal brush is a round brush that is fatter at the base, relative to the length of the bristles, but still tapers to a nice sharp point. This gives us fuller, rounder teardrops for our petals – and all the other times we paint teardrops.