The title applies to many aspects of the novel, which covers the ninety years of the life of one woman, Hagar. An effective essay could focus on two or three interrelated aspects to which the title seems most closely connected.
On the literal level, the stone angel is the actual monument that marks Hagar's mother's grave. It can be viewed primarily as a sign of her father's love.
Because it is a cemetery marker, it also functions as a sign of death. At various points in her life, Hagar suffers numerous losses of loved ones. At the end, she herself dies. Which of the others' deaths affects her the most? How does her attitude toward death change as she feels it grow near?
The material of stone is also significant. What aspects of Hagar's life show her or other characters to be cold and unyielding? Was she trying to hide her feelings toward them, especially those who died?
An essay could highlight two contrasting instances, such as her attitude toward her sons, as one of them died young. Another approach would emphasize the emotional growth (or lack of it) in the stony-hearted woman. The novel offers rich material to work with.