A Pocket Dimension is a very secluded realm of reality. Different dimensions are only briefly explored throughout the main series.
Layout
Pocket Dimensions are considered specialized realms or worlds within another being through magical means. The exact look of said Pocket Dimension seemingly is different between individuals:
Lion's Pocket Dimension
The pocket dimension within Lion's mane mainly is presented as a endless savannah of pink grass, with a singular hill at the center with a tree growing on it. This area specifically has been a storage for both Rose Quartz and Steven Universe.
Spinel's Pocket Dimension
While fused as Spinel, the pocket dimension shown appears to be a mixture between Pearl and Lapis Lazuli: it has dull green/gray broken down pillars coming out of the sky, containing what appear to be gem chunks inside. It has rose bushes growing along a stone path and a dull light purple sky.