Quote:Below is a list of rocks that are safe for aquarium use.
Safe Rocks:

Crystalline Quartz ( also known as Rock Crystal, Amethyst, Citrine, Rose Quartz, Smokey Quartz)
Granite (care must be taken with granite, some forms contain high concentrations of pyrite)
Jade
Microcrystalline Quartz, also known as Quartzite and its metamorphic forms: Jasper, Agate, Chalcedony, Sard, Carnelian; Green coloration should be avoided due to presence of copper)
Onyx
Petrified Wood
Slate
Basalt
Porphyry
Schist
Commercially available “River Rocks”
Mica
Tapecrete or similar acrylic based concrete products, properly treated and sealed
Obsidian

Unsafe Rocks:

Coal, as a naturally occurring hydrocarbon, this rock is often contaminated with many other hydrocarbons.
Lava Rock, not recommended due to a multitude of sharp edges and the possibility of high sulfur and hydrocarbon concentrations
Coral, causes alkalinity issues
Dolomite, the metamorphic form of limestone
Fools Gold also known as pyrite (or any other rock with metallic veins - Acid Producing and even more so, often a lead hazard)
Any rock with a green or greenish color to it, these are most commonly contain copper)
Fossils, with exception to those in Slate
Limestone, the sedementary form of coral
Manufactured Quarts Crystals
Marble, if not sealed with a high quality aquarium safe epoxy
Sandstone
Shale as it often contains hydrocarbons that can be harmful in a captive enclosure
All Metamorphic and Sedimentary Rock not on the safe list