Because the number four is commonly associated with material things—the physical elements, the directions of world, the seasons of the world—both squares and crosses are often used as symbols of the material world itself. The five-pointed star, also commonly called a pentagram, has been in use for thousands of years by a variety of cultures.
For example, a point-up triangle might represent a strong foundation or stability, as it is rooted to the ground through a solid base.
The elements of earth and water formed from point-up triangles, even thou these are the two more material elements.
As a three-sided polygon, the triangle represents the number three, which is meaningful to many groups.
As such, triangles and other symbols made of three parts may be used to present such concepts as past, present, and future or spirit, mind and body.
Circles are frequently used as sun symbols, as well as representing things associated with the sun.
The astrological symbol of the sun is a circle with a dot in the middle.
This comes primarily from two associations: the four physical elements (earth, water, air and fire) and the four cardinal directions (north, south, east and west).
Indeed, the astrological symbol for Earth is a cross inside a circle.
The ouroboros is a circular symbol created by a creature feeding off it's own tail, or else two creatures feeding off each other's tails.