This started as a bubble of gas.
Lava cooling near the surface traps pockets of gas, and those pockets set into the rock as empty rounded voids. Much later, water carrying dissolved silica works its way through the rock and finds the void. Silica comes out of solution onto the cavity wall and starts building inwards.
That is why a geode reads backwards. The outer rind is the oldest part, the first material to be laid down, and the crystals pointing into the middle are the youngest. The hollow at the centre is simply where the water ran out before the job was finished.
A solid one would be a nodule. The gap is the whole point.
Mineral: Quartz (silicon dioxide, SiO₂), a chalcedony rind with druzy quartz inside
Crystal system: Trigonal
Origin: Rancho Agua Blanca, Tabasco Municipality, Zacatecas, Mexico
Hardness: 7 on the Mohs scale; hard, though the rind is thin and will crack if dropped
Treatment: None
Dimensions: 2.3 x 0.82 x 2.7 cm
Weight: 8.86 g
You will receive the exact piece photographed.