Under the Five Elements Concept (Wu Xing) in Chinese metaphysics and Feng Shui, each element is associated with a specific shape:
- Earth: Square or rectangle  
- Metal: Circle  
- Fire: Triangle  

For completeness, here are the other two elements and their shapes:
- Wood: Rectangle or column (sometimes elongated shapes)  
- Water: Wavy or curved shapes

These shapes are often employed in Feng Shui design, interior decoration, architecture, and symbolism to balance or enhance specific element energies in a space. 

 

Geomancy.net Colour Reference Chart of The Five Elements