The crystalline voice of Teresa Salgueiro

I was listening to a Madredeus CD the other day, and the thought struck me how often their lead singer's voice is described as "crystalline". Naturally, I know practically nothing about singing and indeed music in general, so this term might just as well have some clearly defined technical meaning of which I am unaware. But anyway, it got me thinking what exactly it's supposed to mean that a voice is crystalline; what does a crystalline voice sound like?

One thing I thought of is that the singing in many, perhaps most, or at least the most well-known, Madredeus songs consists mainly of very long, drawn-out vowels, separated every now and then a bashful consonant or two. Each vowel is in some sense clear and sharp, going on and on like the hard, translucent facet of a crystal; its constant frequency is like the constant straight surface of a facet; then a consonant, i.e. an edge separating two facets; and then another vowel, different from the preceding one, just like two facets of a crystal may differ in shape and be at angles to each other.

Not that there's anything wrong with having a crystalline voice, of course; I like Madredeus a lot, although I cannot help agreeing a bit with those who have described their music as "diluted"; a very felicitous choice of word, I think. Not all of their pieces, it is true, but many, sound like some sort of ambient music, something that can be played unobtrusively in the background while you dedicate yourself to work or whatever else you have in mind to do at the moment. Much of it hardly sounds like something that one would want to explicitly listen to. Maybe the problem is simply that a thick-skulled, thick-skinned boor such as myself needs the kind of music that hits hard straight in the face, and keeps on hitting until at least something of it seeps through to me.

Anyway, I guess the reason why this whole crystalline voice business has been getting on my mind at all is that I am wondering if "crystalline" really is a characteristic of a voice by itself, rather than of the particular combination of voice and the songs they sing. That is, if the same singer sang something that consists of normally flowing text (preferably something where meaning plays a prominent role), rather than being more or less a sequence of very long vowels each melting tastefully into the next one, would her voice still be described as crystalline? Especially if she sings in a language such as Portuguese, which is surely the last that could be accused of treating its vowels with circumspection, and whose abundant sh-like sounds are about as crystalline as mud. Consider the fado singers for example; surely Amália Rodrigues, or some of the younger ones for that matter, Mariza or Mafalda Arnauth for instance, surely these are fine singers with good voices; but just as surely, no matter how they sing, as long as they sing the kind of songs they sing now it wouldn't occur to anybody to describe their voices as crystalline.

P.S. Querying Google for Teresa Salgueiro crystalline returns a curious mixture of results; some refer to Madredeus, and some contain publications of Portuguese and Brazilian chemists and physicists. :-)