I would like to know if there'd be any way to forge a metal with an amorphous natural glass such as opal or obsidian into an alloy. Could such a thing be crafted into tools? Would it be conchoidal cleavage or a ductile split?
It sounds like “pig iron”.
The inclusions of stuff that’s not metal makes it brittle and is considered poor. Great effort goes into getting rid of that stuff!
Note that obsidian is glass — mostly silica — formed by melting certain minerals in lava. The smelting pot pretty much does the same thing! So pig iron is exactly whay you are describing.
The properties of an alloy, mixture or compound depend on its constituents, the chemical and physical bonds formed between them and on the process used to create it. Because an alloy contains a metal that is hard, shiny, tough, brittle or ductile does not mean that its alloys will “inherit” these properties, although they might, depending on circumstances. Obsidian and opal have a variable composition although are made mostly of silicon dioxide, so it would probably be easier to use sand.
A lot would depend upon the process used to mix them. The original amorphous structure of the obsidian or opal would be totally destroyed by any extreme heat process and a new structure would be formed depending on the proportions, the presence or lack of other impurities, the temperature reached and the method of cooling among other things.
High silicon content iron alloys tend to show excellent corrosion resistance, however they also have low strength and impact toughness, high hardness and brittleness, poor welding ability, low thermal conductivity and high thermal expansion coefficients leading to casting defects and an inability to withstand thermal shocks. Iron – silicon alloys are often useful as magnets
As a side note the irradiance seen in opal is caused by the presence of 6-20% water trapped in the crystal structure. Extreme heating of opals will destroy this irradiance.