yes, there are differences even though they're both volcanic in nature, pumice is more of a "glass" than scoria. Pumice will often float when in water; scoria does not. As for the differences in mineral composition, a quick internet search will provide the breakdown for you. What i wanted to let you know however, is something that has not been given much notice when it comes to using scoria (red lava rock/pebbles). Scoria can damage the roots of certain plants so be cautious when using it within your mix, even the 1/4 inch or smaller sized rocks as they have a very coarse texture and can potentially be abrasive on roots. When roots are actively growing they can scrape against the scoria and expose the tender tissue which could lead to root rot or expose it to fungal infection. Just an FYI : )