Hi Rose,
The Moh's scale of hardness is a little trickey as it does not give an "absolute" hardness but rather a "relative" hardness.
In other words on an "absolute" scale Talc is 1 and diamond is 1500.
On the Moh's scale Talc is 1 and diamond is 10 leading you to believe that Diamond is 10 times harder then Talc.
The table below shows the 'Absolute' hardness of the minerals used by Moh's in his scale.
Hardness Mineral AbsoluteHardness
1 Talc (Mg3Si4O10(OH)2) 1
2 Gypsum (CaSO4·2H2O) 3
3 Calcite (CaCO3) 9
4 Fluorite (CaF2) 21
5 Apatite (Ca5(PO4)3(OH-,Cl-,F-)) 48
6 Orthoclase Feldspar (KAlSi3O8) 72
7 Quartz (SiO2) 100
8 Topaz (Al2SiO4(OH-,F-)2) 200
9 Corundum (Al2O3) 400
10 Diamond (C) 1500
So,,,,How does this help us to answer your question ?
If you try to polish rocks (agate hardness #7 ) with the (sodalite hardness #5.5 - #6) the sodalite would finish looking very dull.
We could get even deper into the absolute hardness of a given material but for tumbling I don't think it is necessary.
One thing I would like to mention is that in my experience I have found that the same "rock" can differ in hardness.Petrified wood is a good example.
I'm not a chemist or geologist and don't know the difference between petrified wood,agitized wood,opalized wood or replacement wood.
I "have" tumbled all of them and had mixed results.The only way I can explain it is that it seems to me some of the wood didn't get fully petrified and left soft areas in the grain that won't polish.
Are you getting as confused as I am?? LOL
Instead of me explaining all this technical crap let me say it this way.
In a lot of cases mixed batches of different hardness give mixed results. Now I can understand myself!! LOL
If I were to teach a tumbling calss to people who have never tumbled before I would insist on them starting with agate and or jasper of "good" quality.These 2 materials can't be beat for consistantly good results.
The reason for "good" quality material is to avoid the problems that the poor quality material will cause.
The reason for agate and or jasper is because they are both hard materials.The harder materials will give a consistant shine where the softer materials are nothing but trouble for a "newbie".
One point on mixed batches.As long as the materials are somewhat close in hardness you "can" rough them together.You'll have ti keep an eye on the softer material as it will erode faster then the hard will.
Tumbling your first load of one material is a tuff row to hoe.You have some rocks that are ready for the next step ,,,,,,but not all.If you cull your rocks you won't have enough left to start the roughing again.
There's a few ways around this but the cheapest and easiest way is to add to the rough you have by putting in pea stone or other small processed rocks.The actual type of material the processed rocks are made up of differ with location.In other words if you live in Florida,,,forget it,,,,,,but,,,,,,,,,if you live in other parts of the Country where harder material is used you may even find some "keepers" as I think it was Cookie had mentioned.
I've read here where some have used plastic pellets in the roughing stage.Don't do it.The pellets are used for a cushioning effect and you DO NOT want that in the roughing stage unless you are roughing some of the softer or more brittle materials.
My index fingers are starting to blister so i'm going to go and soak them!! LOL
Hope this helps a little.
Tom