Hi
MMM 
You don't believe in loaded questions, do you? LOL
I've been sitting here trying to determine how to answer this, and I think I will do it by example.
But first let me say a few things
Firstly, I can't speak for the others who make and sell, or plan to sell their rock. One would hope that we would all make rock that we ourselves would use in our tanks (see my backwall). So how they justify the materials vs risk is going to be unique to each person. I stress the risks to the LFS, and all have shrugged them off - they feel the chemicals, etc that we add to our tanks are a worse risk then the possible contaminates in the rock.
That being said...
We know that manufacturers of building materials, etc. have to list the properties of the product for safety issues. Metals are one that has to be listed, even in trace amounts. So reading the MSDS for any product you are considering is something you must do if you want to cover your butt. Failing a MSDS for that particular product (say limestone from a particular quarry), you can always contact the plant that makes the product and ask for the elemental breakdown of their product. If they won't share that info, move on to a different source. If you try to stick with things that are used for animals, you will have the fewest implied risks. But just because something is considered safe, doesn't mean it always is.
For example, much of the Caribbean beach sand and crushed coral sold in the hobby is shipped via barge or cargo ships. Let me tell you about my experience with something shipped via a cargo ship.
A few years ago, when I bought my pony, Storm Cloud, I needed to buy a saddle too. I had a couple of occasions to sit an Australian saddle, and really liked them. So I shopped around and decided to go with a place that imported them, as well as custom fitting the saddle to the horse's withers. I order the saddle in, wait a month for it and when I get it, I notice an odd smell. But the saddle looked good - great in fact and so I introduced the saddle to my untrained pony, and proceeded to start her training. About 2 weeks in, as we are doing some ground exercices, she spooks and totally freaks out. She is galloping like mad and bucking, and the saddle slips a bit to the side, which scares her even more. She hits the side rail of the corral and I hear a horrible sound - I'm sure my beautiful pony has broken something, but no, she keeps running and bucking and shows no sign of injury. I finally manage to get control of the situation, get her stopped and access the damage. My pony is fine, just scared, but my $700 saddle is trashed. I'm freaking out. I bring Storm to the barn and get the saddle off and after seeing to her, examine the saddle. The entire inside, the wood, the stuffing, everything is covered in mold and the wood was showing signs of rot. And you can see water stains on the leather on the inside of the leather. To make a longer story shorter, let's just say I contacted the seller, who made it right, but explained that the saddles ship by cargo ship from Australia, and go to a factor in NYC. Seems the factor thought that no one would notice a shipment of bilge-logged saddles, so he/she/they must have paid to have them surface cleaned and shipped them on to the saddle seller, who, thinking they are getting good wares, just stuck them in the warehouse until they ship to customers. The point of this story is to show that even when the materials are good, improper handling can ruin or contaminate them. A lot of shipments today are handled by independent truck drivers and cargo ships - these guys aren't cleaning their trucks and holds between loads of materials, or at least a lot of them aren't - they are just trying to get from "point a" to "point b" and get their money. And that doesn't even start to go into in plant contamination, what with lines breaking and possible chemical contact, etc.
What I'm trying to say, I guess, is that life is risk. I could die the next time I step outside. I might try a new product in my tank and nuke the whole thing. You just really never know. You find the best materials you think you can get within your budget, and move on with life. I'd be more worried about the RO/DI water that many of us buy from our LFS then I would about the possible contaminants in my MMLR. LFS are notorious for not changing filters regularly - not all, but quite a few are guilty of letting them go too long before changing them.
Ok, so on to the example.
Take a 10lbs MMLR rock, made to the Ol Skool recipe. In that 10lbs rock will be something close to 3lbs of cement, 2.5lbs of sand and 5lbs of "Other" - OS or CC. Now let's say that I used the worst possible (most contaminated) materials I could find. The cement, in all honesty is probably the most "dangerous" thing you will be using. The sands/grits
may have trace elements that are unwanted, but if the MSDS doesn't list them, then they are so inconsequential as to be almost non-existent. Especially in light of us using 2.5lbs. Even if 1 full percent were present (and at that amount, they have to list it), you would be looking at a total of maybe 11 GRAMS of unwanted material. Plants across the world make their portland from what they can get either locally or cheaply, and this can vary from lot to lot within a plant, based on what is available. Now we know that some metals react with cement and are in essence, broken down or converted. Aluminum is one. Aluminum is toxic. Aluminum is present in all portland cements, in a fairly large portion - generally around 20% are aluminum compounds:
In this example, 20% would be about 272 GRAMS of Aluminum Compounds. However, the aluminum is somewhat converted in the hydration process, and rendered "safe".
And notice the "other" - *** is "Other"? Who knows. Could be slag from smelting, or just about anything else. Phosphates are often mined from the same locations as the other materials used to make cement, so phosphates could be a part of that "other".
Sands are easier to screen for impurities. Holding it in your hand and examining it will often tell you if you need to worry about that sand in particular. Look for darker/lighter bits in the sand that don't look like the sand itself. Sometimes a magnet will pick out them out if the metal is ferrous, but ferrous metals aren't really something I feel we need to worry about as they tend to corrode when in contact with cement and then are either incorporated into the cement matrix or can be flushed out somewhat in our water baths. So let's say again, that we figure 1 full % for impurities, and we get around 22 GRAMS. So all told, we are looking at close to 300 GRAMS, or just over half a pound of total
possible contaminates. That does sound scary, I know, but these are numbers I'm pulling out of my butt, as most products we use don't list metals in their makeup, so one has to hope that that means they are present in such minute amounts that they make no difference. And then, the nature of the cement will "bind" much of the possible bad stuff, and some will be flushed in the kuring process.
If you are considering a product, use Google - it is your friend. See if others have used it in their tanks, fresh or salt - if it has been, chances are it is safe. For the most part, you can assume most stuff sold in the US, with a list of components that are safe, will
be safe. And if you just can't shake the worry, then don't make your LR, purchase the real thing...
Seems the MMLR movement is older then I even knew - Paul B., a RC member, has been making rock since the 60's, and has a tank that has been going since the 80's. He made a lot of his rock and hasn't had problems. Many aquariums have been using MMLR and cement "shot-crete" to form the backbones of their displays, with no problems. If it makes you feel any better, I have never heard of a tank failing due to the MMLR - though I have heard complaints about nitrates and excessive algae growth; both of those can be as easily attributed to the hobbyist's habits, as it can the LR itself.
I've tried to answer this the best I can. I'm sure many will have things to say, or to add, or to contest, but really, I think we have enough to worry about in life, and this isn't really one of them, but is something one should be aware of.
