Piedra de toque

Piedra de toque
 

Literally, a "piedra de toque" is a hard rock where you draw two lines, one using a rod of good gold or silver and the other using a ring or another object you want to try. When you add acid to those lines the change of colour tells you the quality of those precious metals by comparison.

But we use the phrase figuratively to refer to the element that let us sort out good/right and bad/evil/wrong, that is, some key factor that allows to categorize a thing or a circumstance. The sense is not only material but mainly ethical or spiritual.

In a nutshell, "piedra de toque" is what we use to determine if something is good or bad, right or wrong, good or evil, useful or junk. It's a key element to a diagnosis, that's why people use to mix it up with a key element of the thing to be diagnosed (they mix up "piedra de toque" with "piedra angular" -cornerstone- and "clave" or "dovela" -keystone-).

How can we express the same notions in English?

A literal translation works: touchstone. It has 76 entries in BNC, although a few of them are references to the character by that name in one of Shakespeare's plays.

In some specific contexts, perhaps a shibboleth is one possibility.

Thank you both. I couldn't find it in my favourite dictionary so I "panicked" instead of consulting another one.

I didn't know this term shibboleth. It sounds a little bit discriminatory. I can't think of any use of "piedra de toque" that might imply inclusion/exclusion.

A little far afield, but maybe the gold standard?

Oxford gives
piedra de toque (en joyería) touchstone, standard; (muestra, punto de referencia) touchstone

Granada University bilingual gives a good example,
The touchstone for professional practice are the professional codes of ethics that govern medicine in face-to-face relationships with patients.

Moliner gives,
P. de toque. 1 Roca silícea, generalmente de color negro, que se emplea para valorar la riqueza en oro de una aleación, por el color que ésta deja al frotarla contra un trozo de ella. Þ Lidita. 2 Cosa o situación que sirve para *probar o confirmar cierta cualidad, sentimiento, etc., de alguien: ‘Esa será la piedra de toque de su vocación’.

lidita1 f. Variedad de jaspe; se emplea para contrastar el oro.

Oxford (again) gives (for shibboleth)
a (idea) dogma m
b (distinguishing characteristic) (liter) rasgo m distintivo
c (password) santo m y seña

I agree with Perikles, in that, in one way "shibboleth" could work in specific contexts.

And yes, "gold standard" may go far afield, but it's a good option to have in some metaphoric context...

As usual the proof of the pudding is in the eating of it.
And you know if someone is Argentinean and not from Nicaragua, not in the "voseo", but in their lexical choices... ain't that right? ;)

Sure, it is! :D

Thank you for that information, JPablo.

You're welcome, aleCcowaN! :D