A Memory Stone (記憶の原石 Kioku no Genseki) is a crystal that has a memory contained within it. The stone's color indicates the person to whom the memory belongs.
In XBlaze – Lost: Memories, Memory Stones of "Me" (white) and Es (blue) are assembled from the Memory Fragments and unlock the main story. Stones of side-stories are the riddle prizes, and their colors are green (Hinata and Yuki Himezuru), purple (Mei Amanohokosaka), red (Kuon Glamred Stroheim) and black (Tōya Kagari).
The memories that are released can be observed by going to "Memories" in the "Information" menu option.