Tweet,,, Hermatite gave you some good advise and I'd like to add to it.
Some landscape suppliers buy their "river" rocks in large wholesale lots from who knows where.
I live in CT and if they came from here they would be worthless for tumbling.
If you live in an area where good tumbling rocks can be found you can either collect them yourself in a stream or even better go to a rock processing site and ask if you can look around for tumbling rocks.
It's hard to ID rocks when you first start out and especially if you self collect them.There's nothing more gratifying then polishing rocks that you have self collected but as a newbie the only way you'll learn which are good and which are leaverites is by trial and error in your tumbler.
This,in most cases,will cause you a lot of dissapointment,time and money wasted on grit and polish.
That being said,there are a few ways to tell if a rock is worth tumbling.You can do a scratch test using a small pocket knife,a sharp piece of quartz or a harder material is a broken file.
If you can scratch the rock with the above it's a leaverite if not it's a keeper.Now I MEAN scratch it not just get a white chalky looking mark.
OK ,,,,here's what I suggest.Don't buy landscape rocks or river rocks.I know they're cheap and very easy to obtain but again you're new at this and you should stay away from avoidable trouble.Buy your rocks from a local rock shop,on line,here,or where ever you can so you know exactly what you are getting.I again suggest you stick to agate or even better jasper.Most common types of jasper are fairly cheap and if you don't get good results you'll know it was you and not the jasper.This alone will eliminate many headaches for you as a newbie.
If you need more info on this just ask and I'll be more then happy to help.
As for how to ID different types of tumbling material.
If you go to GOOGLE.com and click on the Images link you can type in almost any kind of tumbling rough on the planet and GOOGLE will link you to the photo's of them.
Here's the link to GOOGLE Images:
www.google.com/imghpI have found this to be a great resource for not only identification but also for pricing.
If you can't find a photo there in the Images link you will see on the top of the same GOOGLE page another link ,,,
"Web"
Leave whatever you typed in the little box and just click on the "Web" link and go from there.You will see a ton of listings for whatever you are searching for.
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I just went to the Image link and typed in jasper rough.
Here are the results I got:
images.google.com/images?q=jasper+rough&hl=enAgain if you need more help just ask away,,,,,I love to help,,,,,
Tom