I am looking at one as I type and in that tank I have 2 stones. One is limewood and one is an el-cheapo walmart .99 cent-er. They are blowing identical sized bubbles. The limewood was all I had at the time I grabbed it so in it went. I don't think you see a difference with them until you get into salt water, iirc. The stone diffusers clog faster though.
i was using the blue mist fine air stone...but they crumble when touch after a couple month...litterly crumble in your hand into tiny sand..
I want to try the limestone because i don't want to spend .69 for a single pack and keep changing them..