Would you allow a magus to take Steel as a Minor Magical Focus?
I'd be tempted to say yes, given that "Metals" is a canonical Major Focus.
I see no reason why not
Bob
I don't see a big problem with mMF(Steel). mMF(steal) on the other hand is a huge can of worms.
What about stele? 
I first thought, "depends on how people tend to arm/armor in your campaign," but on thinking about it more, steel is narrow enough to be a minor focus; even if it shows up all the time, it's just one thing.
Hi,
Steel is a good minor focus. I'd be even happier with Iron, which includes steel.
(Since it is likely to come up, I'd recommend totally ignoring the conversation about how 'carbon' relates to steel, since carbon does not exist in ME as such.)
Anyway,
Ken
I'd personnaly say no for this reason: metal focus mainly contains... iron/steel. Other metals are quite scarcely used : gold? silver? when do you cast a spell including those? one or two time a year, and penetration may not be an issue.
However, steel? that is your knight opponent armor, its weapon, the bars of your prison, etc.
IMO a type of metal, such as "titanium", could be a more adequate minor magical focus based on metals. Your magic would then revolve about that type. (And, if "iron" would be a focus which would be okay, weapons should not, normally, done in iron - I do not presume of historical weapon creation, which i know nothing of, I'm only speaking in game balance terms.)
If metal is mainly about steel, then steel only shouldn't be a minor magical focus: precious metals are too rare to counterbalance this distinction.
Earth and stone are IIRC major focuses too. So should they because they are large such as fire would be in ignem. Sand (or dust) would be a minor because earth is not only about sand; "granite" would be ok for a minor focus on stone.
Copper, Bronze, Brass have been used for a number of purposes historically.
What Tellus said, and one shouldn't rule out a focus based on the possible things that it might affect. Yeah, a steel focus can hit a lot of targets, but it will also miss a lot of targets. But is a focus on steel narrow in scope? Yes, yes it is. Then it is a minor focus.
Actually, a large number of metals and their alloys have been used historically: silver, gold, lead, pewter, bronze, brass, tin, copper, to name just the first that come to mind. Ultimately, a Minor Magical Focus should be "slightly narrower than a single Technique + Form combination": and I think that steel encompasses less than a fifth of Terram. Still, the usefulness of it as a focus is the reason why I asked, and the reason why I think it's best to leave plain cast iron (which would include most medieval prison bars!) out of such a focus.
Hi,
Metals also include things like weird supernatural metals, such as the titanium in Ezekiel's chariot 
, aside from the already-mentioned lead, brass, bronze, copper, silver, gold, electrum, platinum, pewter. Probably not mercury.
Iron is fine. Steel... I'd rather forgo the arguments about how steely iron must be to be steel. Yes, iron is probably the best metal. Still minor.
But, as usual.....
Anyway,
Ken
Is it narrow?
We are of course in the "YSMV" line of thought, but steel is armors, steel is furniture, steel is weapon, steel affects almost any item enchantment, steel is even building if you are dedicating. If you are able to do all of those things, and double your lowest art..; you even can find afamiliar (made of steel I mean) if you look anywhere appropriate.
I'm really not sure that "healing" is so large, nor ward, which is already one of the largest minor magical focus I know of (because ward only ward...)
It brings back the whole "does a wolf minor focus affect anything in the shape of wolf, even if not a wolf?" debate which I read on those forums, but without the shape problematic, but the more fundamental matter problematic.
You can even clothe yourself with steel if you are a bit specialised. Why even bother with other things.
Compare that with your brass thing. You cannot really build, you are not gonna affect most mundane nor even magi talismans. But which magus has no steel in his talisman? dagger are a broad talisman type, jewelry (circlets) often contains steel...
Well whatever, if asked, it would be a big no in my saga as a minor. But you know, YSMV 
.
And as a side note: if "what it encompasses is no matter", then why even botter with 10 virtues and flaws? I thought that numbers help define boundaries and limit players to keep their character somewhat balanced toward the system. Why if it doesn't matter, shouldn't we have a magus with a focus in damage for example, and a focus in fire?
Steel production in the Middle Ages isn't so high that they are building with it...
So what if your talisman has steel in it? You only get a focus bonus when enchanting. You don't get a bonus for adding additional pawns of Vis for capacity.
On the one hand, iron/steel is a big part of the "metals" part of terram, as it is the substance used for weapons, armour and tools.
On the other hand, "dead wood" is given as a minor Herbam focus example in the ArM5 main rules allowing Herbam magi who specialise in wooden stuff rather than living plants to excel quite cheaply. I'd allow it in my saga, but if you get suspicious and start laying down rules about what "dead wood" allows for a Herbam specialist, you'll probably want to be careful with "steel" focus.
On a related note, I once was suggesting "Sapphire" as a terram focus because I wanted a mage who would create huge blocks of gemstone and turn them into furniture. CJ kindly allowed me "gems" as a minor focus, as he reckoned it was such a tiny part of Terram it wouldn't make a difference.
I wonder what kind of stats a diamond sword would have...
Hi,
Bad stats.
Hardness isn't everything. Weight matters, as does flexibility.
Anyway,
Ken
To be specific, diamond hardness referes to diamond's resistance to scratch. It does not mean it is indestructible nor resistant to shock.
Specialists (gemmologist) use the word "toughness" to describe the resistance to shock. In this case nephrite, then jadeite (or jade) is the toughest stone/gemstone existing: some civilisations made knife, axe and hammer out of it.
Diamond has a cleavage plane, which means that hit under a certain angle, it will just break in two pieces.
In this aspect, corindon (saphire, ruby) is better than diamond: its hardness is only second to diamond, but it does not have a cleavage plane, thus has a much higher toughness.
On the other hand, it's not entirely clear how "Mythic" diamonds, rubies and sapphires would behave. Diamond comes from a greek word that means "untameable"; in the ancient and medieval world its hardness was well known -- but I am not sure that the distinction between hardness and toughness was clear to the ancients.
And truth be told, Perseus used a diamond sword to cut Medusa's head, and Cronos a diamond sickle to castrate his father Uranus.
True!
However, in keeping with the way AM5 (very cleverly!) handles the distinction between real things and mythic things, the sort of diamond you would need to create a sickle or sword of that kind would be a Diamond of Virtue. An ordinary diamond would just not do.
I don't know about this. I follow the argument (though looking at fire within Ignem is always problematic because Ignem is sooo limited). However, what must also be considered in the argument is the scale of "metal" already. Is "metal" just barely too big for a Minor Magical Focus, is it in the middle, or is it pushing the limits of a Major Magical Focus? I would argue it's actually on the barely too big for a Minor Magical Focus end. Here is why. For the breadth of a Minor Magical Focus, you should have under 20% of the whole Form (if restricted to a single Form) or its equivalent. What does Terram cover? Metal (iron, lead, bronze, silver, gold, etc.), stone (granite, basalt, limestone, etc.), earth (sand, soil, etc.), gems (rubies, sapphires, etc.), some random other stuff (glass, for example), and partially just solid things (though those may need requisites, too). I would not rate some of the last ones on the list as broad as the first three. If we lump gems, random other stuff, and just solid things together we probably have roughly the breadth of the others. That would make "metal" about 25% of Terram. For a Major Magical Focus, you should be under 100%, though I expect nearly 100% is usually too broad to name something focused enough to be a Focus. 25% seems a lot closer to 20% than to 100%. It doesn't take removing much to drop that 25% to under 20%. Let's say iron/steel account for 75% of all metals. That still drops us under 20%.
I'm not so sure about finding a familiar made of steel. After all, we're talking about an animal, and it need not have Might as long as it's connected to the Magic Realm. Any magical wolf could do for "wolf." Plus, "wolf" is actually narrower than necessary. "Canid" is sufficient, allowing for dogs as well. Or if we choose "raptors," then we could have an owl, an eagle, a hawk, etc. I find it unlikely except with an extremely generous SG that there would be a magical animal actually composed of steel. As a result, I still consider "canid" or "raptor" to be much more broadly useful than "steel" (or "iron" - I do agree on lumping it all together for simplicity and clarity, such as "Is cast iron more iron than steel is, or it that the word 'iron' just happens to be in there?")
As for shape/matter, that will commonly show up with a clever player/magus. Turn something into a wolf, then use your other spells. Turn something into steel, then use your other spells. The same thing can be done with tons of different foci. With the HoH:MC rulings on changed shapes, it would seem to me that disallowing anything for this reason would become essentially disallowing many good foci. Sure, "healing" and "wards" would remain. But you couldn't choose any sort of object, being, or material.