Scoria, a dark, glassy rock created from this, is heavier than pumice & has bigger holes
The are medium to large grains 
and you can tell the difference in the dark micas, clearer quartz and the ... liquid 
lava flow, has larger holes than pumice, is slightly heavier and... [DOC] [TLE] Pumice and Scoria [PAR] Scoria and pumice take vesiculation to the extreme.