From Fate/Grand Order Wiki
"Crystal created by concentrating mana under a special environment. The crystallized form of rare mana.
Rare Prism is obtained by burning 4★ or 5★ Servants.
With the Rare Prism Update, 4★ Servants yield 1 Rare Prism and 5★ Servants yield 5 rare prisms when burned.
See Burning for more detailed information.