Malachite's Realm is a location that first appeared in "Chille Tid". This is the realm where Jasper and Lapis Lazuli endlessly struggled for control of their fusion, Malachite, and where Lapis temporarily lost control of Malachite to Jasper.
It appears to have a malachite gemstone texture (light green ripples) with the ground and water as the base. It has a dark green background and a glowing dark sea foam aura, making its occupants' palettes appear teal.
This is where both Lapis and Jasper resided when fused. Its existence potentially revealed that all fusions could have mindscape realms which change accordingly with their personality and appearance, and are inhabited by the Gems that comprise them. However, this could just be a representation of their mind as viewed through Steven's dream.