Macgyvering a Spectroscope
The medieval "nerd" in a D&amp;D campaign might use acids to get minerals into solution.

Sulfuric acid ($\ce{H2SO4}$, vitriol oil) was initially prepared by heating iron(II)-sulfate ($\ce{FeSO4 · 7 H2O}$, iron vitriol) in a retort. The emerging sulfur trioxide was collected an led into water.

Nitric acid ($\ce{HNO3}$) was prepared by heating mixtures of potassium nitrate ($\ce{KNO3}$, saltpetre) with iron vitriol.

Hydrogen chloride ($\ce{HCl}$) was available by the reaction of vitriol oil with halite ($\ce{NaCl}$).

Mixing hydrochloric acid with nitric acid would then give aqua regia.

Another method to get the cations of minerals with low solubility in water into solution is the so called Aufschluß.

Here, one would either melt the mineral with 


a mixture of soda and potash, or 
a mixture of sulfur and potash


in a crucible and eventually leach out the now soluble material with water.



Admittedly, our D&amp;D alchemist still doesn't have a spectroscope :\

In order to characterize the metals in the samples, he could use his Lötrohr, a kind of blow pipe and blow the flame of his oil lamp onto the sample place on a piece of charcoal or a gypsum plate. From the colour of the flame and/or the colour of the residue formed, he could draw some conclusions on the metal.

If our medieval science hero is a bit ahead of his epoch, he would also carry some borax in a bag and create borax pearls for a bead test.