Rubinstein

Rubinstein is the placeholder name of a Crystal Earth pony stallion with a coat,  mane and tail,  eyes, and a cutie mark of a crenelated tower.

Design
He shares his design with "Castle", "Ivory", "Ardent", "Zirconic", "Check Mate", "Crystal Varado", and S03E02 Unnamed Earth Stallion #1.

Depiction in the series
He is one of the nine most frequently appearing Crystal Ponies, along with "Amber Waves", "Autumn Gem", "Castle", "Crystal Arrow", "Fleur de Verre", "Ivory", "Night Knight", and "Paradise".

He appears in The Crystal Empire - Part 1, in The Crystal Empire - Part 2, in Games Ponies Play, in Magical Mystery Cure, in Flight to the Finish, in Trade Ya!, in Equestria Games, and in Twilight's Kingdom - Part 1 and its recap at the beginning of Twilight's Kingdom - Part 2.

Appearances
Times are approximate.

Storybooks
He appears in Welcome to the Crystal Empire!