Sponge Rok and perlite are the same thing-- the first is a brand name for the second. Comes in various grades -- fine, medium and coarse (at least that was the case years ago) It was said: Another option is to check around for "sponge rock". -----Original Message----- From: Eugene Zielinski <eez55@earthlink.net> To: Pacific Bulb Society <pbs@lists.ibiblio.org> Sent: Tue, Oct 6, 2009 5:48 pm Subject: Re: [pbs] Well-drained growing media (was Oh No! A Boophonedisaster!) Another option is to check around for "sponge rock". This is large sized perlite that some orchid growers use in their growing media.