The symbols in The Glass Castle are:
the glass castle - her Dad planned to use his knowledge in science and math to design a building made of glass, the roof of which would have solar cells that would capture the sun's energy to convert it to energy for air-conditioning, heat and electricity. It was the family's dream home and her father worked on the plans for years. The symbolism is that glass is seemingly transparent and empty, like his plans. It sparkles and looks pretty, but is impractical for a home, particularly in the desert where the solar heat would be oppressive. When he talked about the glass castle, her Dad created a sparkling picture in their minds about what their home would be like, but it was an empty dream that would have been impractical even if he did achieve it.
the desert - Deserts typically have sparse terrain, devoid of the massive colors and variety of plants and flowers found elsewhere. It was symbolic of her life, in which her family had few possessions, even things like furniture, let alone excessive trappings such as fine china, tablecloths or electronic equipment. The desert has very little water, and her family usually had little access to water, since they often didn't have running water in the places they stayed.
-the cactus: Jeannette said, "We were sort of like the cactus. We ate irregularly, and when we did, we'd gorge ourselves."
In contrast, the grandmother's house in Phoenix represented civilization and restrictions, which Jeanette's parents rebelled against. The house was what most people would consider "normal." The Walls didn't like staying there and their visits were usually shortened by quarrels between Jeannette's parents and grandmother.
the Joshua tree: her mother stopped to paint, which resulted in their staying to live for awhile in Midland. A Joshua tree is resilient, and twists and turns whichever way it needs, in order to survive the elements, just as the Wall children did whatever they needed to do to survive in their neglected environment.
rocks- which traditionally represents a solid foundation, permanence, something that isn't easily worn away. Whenever the family pulled up stakes and moved (usually in the middle of the night), in an effort to travel lightly, each child could bring one object. Jeannette chose her favorite geode from her rock collection. She loved to explore the desert for interesting rocks and always hoped to discover gold. Instead, she often found iron pyrite, "fool's gold." Jeannette had very little permanence in her life, so a rock was a symbol of what she was lacking.
The archetypal symbols in the book are: the sun in the desert, which represents the creative energy of Jeannette's mother in her painting endeavors and Jeannette's writing talent; and rocks, which represent Jeannette's longing for something solid and stationary in her life.