No, they are not. The meaning of an idiom is not predictable from its constituent words, that’s it. A metaphor involves some kind of meaning transfer. In this c

No, they are not. The meaning of an idiom is not predictable from its constituent words, that’s it. A metaphor involves some kind of meaning transfer. In this case some properties of the ‘prototypical’ Holy Grail (desirability mostly) are transferred to the stalagmites. The source for the transfer itself could potentially be an idiom, but in this case it’s not, actually. The word ‘grail’, albeit rare, has a normal meaning (a kind of cup), and ’holy grail‘ is what it is: a holy cup. We just decided based on the strength of the literary tradition to call a particular object that way.