* Circular: Round or circular
* Square: Square
* Triangle: Triangular
* Heart: Heart-shaped
* Star: Star-shaped
* Oval: Oval
* Rectangular: Rectangular
* Curved: Curved
You can also use the more general term shaped followed by the specific shape:
* Shaped like a circle
* Shaped like a square
* Shaped like a heart
Remember to choose the adjective that most accurately describes the shape you are referring to.