Gem (Steven Universe) Wiki

Gems are the fictional alien species that make up a large part of the Steven Universe cast. All Gems have a gemstone embedded somewhere on them, which is the only true part of their body and acts as a brain. The rest of their body is made of magic light that acts "like a hologram, but with mass" and can be altered in nearly all ways other than color. Their forms can be temporarily altered through shapeshifting or permanently altered through "poofing", the process in which a Gem's form is harmed so badly that it disappears and leaves only the gemstone behind. While poofed, a Gem can take time to choose what they will look like when they reform, and whatever they choose will be their default appearance until they are poofed again.