Our original information was goofed? Maybe so! Check out why after the break.
According to the Steven Universe wiki, Sardonyx will appear in Cry For Help, which is episode 63 and the first episode of the 3rd Steven Bomb. Our original source said Sardonyx had been confirmed for episode 64, but it appears that information was wrong.
It seems like the wrong appearance date stemmed from either a translation error, or that the foreign dubbed episodes air in a different order. Looking at one of the tweets linked on the wiki, the voice actress seems to say that Keystone Motel is actually episode 65. This could mean the episode 64 she mentioned Sardonyx would appear in previously is actually not the same episode 64 as here in the U.S.
