Shard fusion unlocks at level 50 and allows shards to be exchanged for other shards. The following charts show which shards can be obtained as a result of shard fusion. Any shard of the correct rarity can be used as an input for the fusion.

https://cdn.discordapp.com/attachments/475565631417221130/475565683732774913/Screen_Shot_2018-08-03_at_1.12.17_AM.png
R -> SR
Fuse 5 R shards for 1 random SR shard
SR -> UR
Fuse 5 SR shards for 1 random UR shard (60% chance) or 2 random SR shards (40% chance)

https://cdn.discordapp.com/attachments/475565631417221130/475565687277092874/Screen_Shot_2018-08-03_at_1.12.54_AM.png
3 R shards -> 1 random R shard
3 SR shards -> 1 random SR shard
3 UR shards -> 1 random UR shard

Note: The SR and UR pools are not the same. For example, Gingerbread shards can be obtained from UR -> UR fusion, but cannot be obtained from SR -> UR fusion.