Maybe some rocks that have rust are aquarium safe, but I'd never trust them. After all, it's not too hard to find another type of rock that is safe to use. Why put your whole tank at risk over a questionable rock?

 

Rust? I'd be surprised if you have ferric oxide. It seems likely that you have a rust colored deposit on the surface. The classic thing to check for is carbonates, using vinegar or other acid and looking for bubbling. Other minerals can become soluble in acidic waters (CO2 injection or peet, or decomposition). These may not mess with the pH much, but can leach trace metals into your tank, for which most of us do not have color tests. The answer will depend entirely on what the deposit really is. Safe bet is to skip it.

 

As mentioned it is not really too common to find actual rust in rocks. One reason is that rust weathers pretty quick and as the rock has been out in nature for a few million years, it is likely to be gone. Far more common to find "rust colored" rocks which are actually other materials like reddish clay deposits.
We often hear mention of rust as a hazard but then we often hear of Bigfoot, too. The metals we need to watch for are the "heavy metal" group. Iron, steel are not among those. The hobby we are in tends to promote hysteria and once written it is there forever but some looking around can show lots of reason to believe the normal metal is not a problem for most tanks. It is not at all uncommon to find things like bolts and screws in tanks. Also it is reasonably common to use galvanized steel tanks as outside ponds for both fish and plants.
Some looking at the gallery on the forum may show some of those. I also use metal screws to hold my wood to the slate.

 

As mentioned it is not really too common to find actual rust in rocks. One reason is that rust weathers pretty quick and as the rock has been out in nature for a few million years, it is likely to be gone. Far more common to find "rust colored" rocks which are actually other materials like reddish clay deposits.

Depends where you live. I grew up in Northern Minnesota where most of the iron in the US comes from. There are rusty rocks there. They have rust on the exposed surface and if you break them in half they are a dark charcoal color. Some places a compass won't work, and metal detectors are useless because they are constantly registering.

 

Our water source percolates through iron bearing sandstone and old weathered Andesite and other ancient lava. It's actually a boon because my tanks do well for Iron supplementation, as long as I can keep the pH below 7.0. What we don't have is decent amounts of other minerals like Calcium and Potassium, our water is really soft being rain water with a lot of tannins from the forest.

 

I've been a oilfield service technician for close to eight years, so I see quite a few "uncommon" steel-types. So you might say I am overly interested in metals and alloys

But in general, no such alloys are in daily use to the consumer/regular household. Just good old SS and high/low carbon Iron.