Fire: a triangle pointing up.
Air: a triangle pointing up and with a horizontal line through the middle of it.
Water: a triangle pointing down.
Earth: a triangle pointing down and with a horizontal line through the middle of it.

– The Four Elements, Angelfire.com

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One of the Oldest, yet still the best, most popular, and most misunderstood Symbolic Representations of the Four Elements bases itself upon two sets of opposing equilateral triangles which come together to form the Star of David. All things considered, most especially the fact that Christian Europe has been using this particular Alchemical insignia since the Middle Ages, one can rightfully claim the entire system of symbolizing the Four Elements is a classic example of the Judeo/Christian Tradition, both then and now. What follows is a brief listing of the most common form of triangles as they relate to the Four Elements of God’s Creation:

The Ancient Triangle Symbols of the Four Elements
1) FIRE: Upward Triangle colored Red
2) WATER: Downward Triangle colored Blue
3) AIR: Upward Triangle colored Yellow with horizontal dash through the upper tip of Triangle
4) EARTH: Downward Triangle colored Green with a horizontal dash drawn through the lower tip of Triangle

In essence, the Star of David provides the most simplistic (and easy to draw) method of representing the various combinations of either Air and Earth, Fire and Water, Air and Water, or Fire and Earth.

Opposing elements are on opposite sides…and are distinguished by the presence or absence of the crossbar within the triangle…Air and earth are opposites and have the crossbar. Water and fire are also opposites, and lack the crossbar.

– Elemental Qualities, by Catherine Breyer, About.com

Also, even Earth and Water may be drawn using a Star of David, in spite of the fact that both of them have Downward Triangles as their official symbols. Why? Because Water is considered lighter than Earth and so becomes an Upward Triangle so that both together still form a Star of David, just as Fire is lighter is than Air and therefore represented with a Star of David when combined.

The Blatantly Obvious Star of David = Earth + Air = Water + Fire
The Somewhat Obvious Star of David = Earth + Fire = Water = Air
The Not so Obvious Star of David = Earth + Water = Air + Fire

References to the Triangular Symbolism of the Four Elements

Each element is either warm or cold, and this corresponds with a male or female gender…The orientation of the triangle is determined by warmth or coldness, male or female. Male, warm elements point upward, ascending toward the spiritual realm. Female, cold elements point downward, descending into the earth.

– Four Elements and their Qualities, by Catherine Beyer, About.com

Each element has a symbol and color. Common symbols are Fire: a triangle pointing up. Air: a triangle pointing up and with a horizontal line through the middle of it. Water: a triangle pointing down. Earth: a triangle pointing down and with a horizontal line through the middle of it.

– The Four Elements, Angelfire.com

The four classic elements are still essential symbols of the cultures and belief systems of many groups, including Native Americans, pagans and various tribal groups throughout the world…During the Greek Hellenic civilization (323-146 B.C.), the symbols used to represent the four elements of Greek physics were based on the triangle:

Fire: Triangle points up
Water: Triangle points down
Air: Triangle points up with a horizontal line through center of triangle
Earth: Triangle points down with a horizontal line through center of triangle

– Colors and Symbols of Four Elements, Feng-shui.lovetoknow.com