what type? are you talking about the kind that comes from the earth (lace rock) and is aquacultured? it's good stuff, i've seen a whole 400 gallon reef with just lace rock, looks awesome and it's good to save the reefs of the world.
Not sure about that.
The following is from their site. (which I dont think I can mention)
All of our Aquacultured Live Rock is light and porious. It is much lighter then typical Fiji, Tonga or Caribbean live rock. You will be able to fill the same amount of space in your tank using a far less amount of our aquacultutred rock saving you a great deal of money. You will also never have to worry about bringing in bad hitchkikers or parasites on any of our aquacultured rock!
Most rock claiming to be "Aquacultured" is really "Maracultured" rock. Maracultured rock is set in the ocean where it gets both good and bad hitchhikers. Our rock is truly aquacultured rock. We seed it in our enclosed systems which totally controls the environment and assure you there will be nothing bad on the rock and only the good stuff!
most manmade rock tends to have unnatural shapes which tends to be there major downside they usually have a donut shape. if it is aquacultered in a store system then yes it is less likely to have bad stuff in it (mantis, etc) but it will also have less variety of good stuff cause you lose critters with time in closed systems
my buddy and I made some of our own with cement and oyster shell... can't tell it apart from the wild stuff now- completely covered in coraline and little critters, best part is you can make whatever size and shape pieces you want.