Just a quick question: p. 97 talks about hard and soft stone, but no examples are given.
What's a hard stone, and what's a soft stone?
Limestone? Sandstone? Granite? pottery (burned clay)?
You must distinguish between mechanical and hydrological hardness.
For a beginner's overview I recommend this:
http://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness
Often sedimentites are softer than metamorphites or magmatites.
If you don't know geology, then you'll have to fake it based on the general characteristics.
Think of soft stone as something that you can scratch away with a dull knife - sandstone, pipestone, limestone, chalk, various hardened clay and sedimentary deposits, etc. It has a good chance of breaking if dropped.
Hard stones are going to need metal chisels to work, and won't break if dropped. Yes, granite, schist, flint, obsidian, gneiss, marble. Most anything that's used for building materials or serious sculpture. If you saw the Olympic Curling, those stones were "hard".
Assuming that I know Moh's scale and am atleast passingl familiar with material science and hardness, that still didn't answer my question, which was specificall in the context of Hermetic magic and enchantments.
This however mostly did, though I'm not sure about the last paragraf - I thought marble qualified as 'soft'?
sorry, I assumed that Mohs, being German might not be so familiar to people with an English-speaking background.
I'll try again. Hope I'm more helpful this time:
Marble is a metamorphite, so its exact hardness can vary (depending on which material was reformed under which temperature and pressure). Marble is based on Ca, so it can easily be attacked with acids (including water, because of the CO2).
In the story, I'd classify marble with those stones that can be easily crafted (think of all those Italian churches).
In 1220, dry and cold things are harder than wet and hot things. This is why a stone from the Sahara is softer than a stone from Iceland.
...the horrible thing is that this actually makes sense... sandstone vs. granite 
but how does this explain clay becomming harder when baked?
Wetness is a bigger factor than coldness?
¿Should ice be a "hard" or "soft stone"?
Soft id say. Its hard but its also brittle.
It is aquam, but if you have to classify it in Terram terms, I would say soft as well: you can carve ice with a knife.
Xavi
"Aquam"? "Terram"?! What does that have to do with the topic?
Unless we've jumped track, the question is about enchanting. There is no other concern (afaik) re "hard/soft stone".
I have to wonder why you are asking - if as an enchantment, neither - it's "water", and I'd be hesitant to believe that "water" would hold much in the way of enchantment. Feels to me like a "1",tho' perhaps ice would be a "2". (But I'd also be curious as to the circumstances where "ice" is going to be prepared with vis to hold an enchantment.)
Otoh, if that's not why you're asking... then why are you?
I could totally see a wizard who focused in ice magic enchanting, for example, a staff of ice as a talisman. If I had to judge the vis capacity of ice, I think probably put it at a 1 or 2 as well.
Ok.
You can think aboaut an Circle of rego Aquam to maintain water freeze and magically "forge" it to becme a sharp ice sword, to overcome some condition, demoniac or something like focusing (posibly with magical focus in ice or other water type). It is mistically possible and very mysticaly attractive, i think.
I never said I had a problem with it - I said I was curious. 
Certainly it's possible - I'm just wondering the story behind it.