Backyard Aquaponics
A place to discuss aquaponics

I am around an ample supply of granite. Is it suitable for gb's?

If you can get appropriately sized granite pebbles for cheap, it should be just fine for AP. The only types of rocks I usually warn against are stuff like limestone/marble that will mess with pH. Otherwise, I use the cheapest gravel I can get, around here that is the 1/2" (and smaller) brown river rock which is mostly quartz based rounded pebbles. Granite costs more down here in FL.

Some people think about how hard the media will be on the hands when digging in the beds. Larger rocks can make that even more difficult though larger rocks are nice for right around the drains. If the gravel is too fine, it will take water a long time to flow through it. A size range between 1/4" and 1/2" seems pretty nice to work with.

To get that small of gravel I would have to use what they call crush and run which has a variety of sizes. In lieu of that could I use the granite as the bottom layer and then smaller pea gravel for the top layer?

Many people do use larger stuff in the bottoms and around the drains of the grow beds and smaller or pea gravel on top. A variety of sizes isn't bad but you do want it to be small enough to make planting work well for you. Try running your hands through some samples to help you decide.

This is pretty out there, but there is a small risk in granite of getting radioactive materials.
http://teamsugar.com/group/1647294/blog/1813710
(note, original source is the NYT)

Granite also can contain heavy metals and other unpleasant minerals.

I would think that river-washed granite would be pretty safe (anything that is going to leach will have already).

This isn't to discourage the use of granite, it's just a heads-up, on the levels of: Hey, look at that meteor! Think it's going to hit u...zzzzt ---end transmission---

Thanks for the heads up. I'm located about 40 miles from the "Granite capital of the world," which means the granite is mined and not river washed if I understand the term.

Perhaps I need to call the Granite Assoc and inquire about it.

all our growbeds have granite in them, either as crushed "blue metal" or as washed river stone

I use Blue Metal (Granite) about 66% and top with Clay Balls, Blue Metal is excellent for promoting good bacteria growth

Same here, all mines granite and quartz thats already passed through a gold mill it's called scats.
Works well for me

So, if the lower limit for size is set by drainage (sand will not work unless one only fills it once/twice daily) is there any upper limit other than for digging with hands? Would would 1" (25mm) be better than 3/8" (10mm) if both were pleasant to dig and manipulate? How about 2" (50mm) polished rocks? I'm sure there is enough surface for bacteria, even if one were to go somewhat larger than even that, and roots and bacteria will act to trap solids, so what is the optimum?

I expect that if you go too large, there will be far more air space/water space and less surface area (especially with polished stones.) You might see more drying out and have difficulty planting seeds directly in the bed as they will fall to the bottom or be washed through the bigger gaps. I expect that you may find the limit on the larger stuff is that it won't be as good at catching solids, will require more water to flood a given size bed, will need more frequent flooding to keep roots from drying out, and may be more difficult for some plants to grow in. It seems for most people the optimum size for the majority of the grow beds seems to be somewhere between 1/4" and 3/4". I like the 1/2" river rock as it is smooth enough to be fun to dig in yet rough and small enough to support plenty of bacteria and of a size that can have larger seeds directly planted in the beds (really tiny seeds like carrots need some extra care when planting.)

I just came back from our local stone store with a ton of #8 gravel. Supposedly it's about 1/4" - I'm not sure how that's measured. I've got about half of one of my GB's with so far. Since I have actually begun construction, I'll post some pictures on the member systems thread soon, hopefully.

My gravel is quarter inch to three eights and is easy to work with. I have planted every type of seed directly in the gravel and it comes up just fine.