The Obelisk only appears in her Gemstone form and as one of Steven's "Guy Man" comics. She is colored light blue and has no known default form.
Shapeshifting: She was shown that she could shapeshift into inanimate objects.
Adventure Time Preview
The Obelisk was captured by the Crystal Gems, only to escape and shapeshift into one of Steven's "Guy Man" comics. The Gems were able to poof her by tearing all of Steven's comics and the Obelisk herself.
The comic was also featured in Volume 1 of the Steven Universe comic series as a bonus.
The Obelisk appeared in "Adventure Time: 2013 Spoooktacular #01" as a preview to the Steven Universe comic series written by Rebecca Sugar.