In Gakuen Alice, if two people exchange Alice stones that they created, it can act as the equivalent of an engagement or promise. Is this based on any specific Japanese tradition? If not, what is it based off of?
I believe this is exchange is a mix of different marriage traditions, the oathing stone and the traditions of engagement rings.
The part about the stones is probably based on an old Scottish tradition with Celtic roots.
The Oathing Stone is an old Scottish tradition where the bride and groom place their hands upon a stone while saying their wedding vows. This ritual tradition is thought to be the best way to express your solemn promise in physical form. The idea is derived from the ancient Celtic custom of setting an oath in stone.
The Scottish believe that an oath given near stone or water, being strong natural elements, make the vows much more binding. Traditionally during the reading of the bride and groom's wedding vows, they hold an oathing stone in their hands, believing that holding the stone during the reading of the vows casts them into the stone.
The exchange is very similar to the exchange of rings that is prevalent is modern culture.
Together this exchange probably symbolizes a literal exchange of vows.