Bombs are blobs of lava, while blocks are jagged chunks of older rock. As magma enters the zone of low pressure, gas starts to come out of solution and forms bubbles in the magma. This condition in which most magma erupts. Solidifying of rocks preserves the holes, which are known as vesicles. Extremely inflated, very light rock froth is pumice.
Hawaiian pumice is called reticulite.
It is even lighter than pumice from other regions, and usually sinks quickly in water. This is a picture of reticulite. This is a picture of lapilii. Lapilii or Pele's tears are glassy droplets, broken from ends of filament of glass drifting away from fire fountains. Bits of liquid thrown up by lava fountain solidify in the air in typical drop shapes.