Quartz is inert. It may cause a sudden outbreak of diatoms (brown algae) because it is a silicate. Slate is inert as is Granite. When using slate, try to use Vermont Slate, the imports from Asia are very pretty but usually have alot of iron in them. Basalt is also good. Lava stone and/or Scoria is also good. I once bought some lava stones used for gas grills on clearance at Home depot in the early fall for about $2 for a 10lb. bag. Granite is nice and comes in a variety of colors from solid grey/blue(sometimes called bluestone) to white with black specs to pink and green. I use crushed granite from a Farm & Feed supply store (like Agway). It is a feed supplement for chickens and other birds. It is some times called Gran-I-Grits or Cix-grits. There are a few sizes so ask to see a sample of each before you buy it. I think the size I prefer is Grouper Grits, about 1.5-2.5mm. It is about $6 for a 50lb bag. If you are unsure of the rocks you are going to use you can test it with vinegar. Take a sample of the rock you want to use and put it in a small container like a tumbler sized glass. Pour enough white vinegar on it until the rock is completely covered (at least a 1/2 inch over it) Wait about 5 minutes. If bubbles start to come off the rock, like it was a carbonated soda drink, then the rock has alot of carbonates in it that will raise your pH. Limestone and Marble will do this. if you plan on using the sample in your aquarium i would soak it well with fresh water with a little baking soda then rinse it again with fresh water. Seashells will also raise pH quite a bit, they are pretty much 100% calcium carbonate. Same with any sand substrates that come from salt or brakish waters.

Granite first choice. I myself am waiting for some more from this construction site in NYC (missed first batch so they said more coming).. has no other coloration in it, but used some from another site that did (rust color and white silica??? that did ok over a year, gave it away though)...

I did some dd on rocks a while back, trying to find the links, here are a few.

(in addition to above excellent information already provided by woeisome)

xxxhttp://www.cichlid-forum.com/articles/rock_metals.php

xxxhttp://www.thekrib.com/TankHardware/rocks.html

Photos of granite sources (mountains)

xxxhttp://www.terragalleria.com/pictures-subjects/granite/granite.all.html

xxxhttp://www.aquariumport.com/Using%20Rocks%20in%20Aquascaping.htm

Not real rock but pretty amazing!

xxxhttp://www.aquariumrocks.biz/products.htm

I had more with photos, will try to find. Best Sherry

Your local pet shop should have shale and/or slate to sell.

Shale is "OK" to use but after a year or more will start to cloud the water. Shale is a sedimentary rock, just compacted mud. Slate is exactically the same thing except it has been heated by volcanic activity (metamorphic rock)so it is much harder and won't disintergrate. A Local Fish Shop (LFS) is a great place to get these supplies and I support them greatly. However, if you go to a quarry, masonary supply or even your own backyard (or someone elses) you can spend far less money to free. LFS may have some other options that look nice in aquariums. Same can be said for a mason or landscaping supply.