Top 日 is the indexing radical — pictograph of the sun with a dot inside marking its radiance. The single sun at the top of the stack establishes 晶 firmly in the family of light and brightness with 明, 星, 晴.
Lower-left 日 — second of three suns. Three suns together depict overwhelming brightness, the original meaning of 晶 as "shining brilliance." Later the sense narrowed to "crystal" — a substance that sparkles as if catching three suns at once.
Lower-right 日 — third of three suns completing the cluster. Triplication is a common Chinese ideographic move for intensification: 木→森 forest, 火→焱 flames, 日→晶 dazzling. The three-suns picture remains transparent: bright beyond bright.
