What was the spiky seed that Hugh Jackman placed on Rachel grave in The Fountain?

The spiky seed that Hugh Jackman's character, Tom, placed on Rachel's grave in The Fountain was a tree seed.

While the film doesn't explicitly state the type of tree, it is symbolic of life, hope, and renewal. This is further emphasized by the fact that Tom planted the seed on Rachel's grave, representing the hope that she will continue to live on in some form.

The film uses the seed as a visual metaphor for the cyclical nature of life, death, and rebirth.