I am sure some of you are way ahead of me on this, but I just heard for the first time the Let All the World in Every Corner Sing by Paul Mealor. I am looking forward to discovering new treasures.

https://www.youtube.com/watch?v=LV5Zg1M9iF0

The Tchaikovsky is wonderful, but, alas, it was composed by a non-Catholic so some on this Forum a priori would not approve of its use at Mass.

The Mealor has a virtuosic organ part, and that factor may lull some into thinking it's a great piece. It's not, IMO. (I'm surprised, MBW, that you think it is. Listen to it a few more times, please, as I will do the same.) Best to stick with the Vaughan Williams. (Oops, another non-Catholic, so disregard that.) Oh, and I almost forgot, the text was written by a non-Catholic.

Sacred (presumably) Catholic choral gems? Sacred (ostensibly) non-Catholic choral gems? Other (possibly) non-sacred or religious choral gems? Modern? Ancient? Big gems or little gems? What are we talking about?

In the realm of little known (which was asked for), perhaps something like the setting by Williametta Spencer of "At the Round Earth's Imagined Corners" (no. 7 of John Donne's Holy Sonnets) would fit someones idea of a choral gem (it does mine). It was the opening piece in this fall's concert by the Ensemble Singers (a group in which I sing), so I'm attaching the concert recording, as well as the following video performance by the National Lutheran Choir at the 2007 ACDA National Convention).

At the round earth's imagin'd corners, blow
  Your trumpets, angels, and arise, arise
  From death, you numberless infinities
Of souls, and to your scatter'd bodies go;
All whom the flood did, and fire shall o'erthrow,
  All whom war, dearth, age, agues, tyrannies,
  Despair, law, chance hath slain, and you whose eyes
Shall behold God and never taste death's woe.

But let them sleep, Lord, and me mourn a space,
  For if above all these my sins abound,
'Tis late to ask abundance of thy grace
  When we are there; here on this lowly ground
Teach me how to repent; for that's as good
As if thou hadst seal'd my pardon with thy blood.


https://www.youtube.com/watch?v=FNbAVihw4Ro

OK, Fr. Krisman, maybe a semi-precious gem. But I like it. The bell part is problematic practically and perhaps also in terms of taste. I also recognize that it is a piece which packs quite a sonic wallop and thus would have to be carefully programmed if it were to be used at all for a Mass.

How about his Ubi caritas/?
https://www.youtube.com/watch?v=KQE4ryqdvMg

It raises another issue that some might want to address. His Ubi caritas is an exact reuse of his Now Sleeps the Crimson Petal from his Rose Cycle.
Same music, different text. Does this, in anyone's eyes, diminish the piece's value for Catholic worship?

I know I've offered this piece before. I can't believe that I conducted my high school concert choir with this wonderful piece.
https://www.youtube.com/watch?v=ifoCMbNu3PM
I would like to have found the recording of this piece, The Rose (Paynter) by Northern Arizona U's Concert Choir.
https://www.youtube.com/watch?v=AEvb1faTbK8
This other setting of the medieval text by the late Gary Garcia. Performed by CSU Long Beach, under Jonathan Tallberg, great scholar/performer. We have done this at my parish and when I had Fresno's cathedral.
https://www.youtube.com/watch?v=mFQb2-8naqM

Thanks, Abbot. That, actually, was what I was hoping you meant.

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https://www.youtube.com/watch?v=jhxrju-2Dro

This one by James MacMillan is accessible and beautiful.

https://www.youtube.com/results?search_query=macmillan+think+of+how+god+loves+you

https://www.youtube.com/watch?v=wfAlAz-zFlU

Sorry, wrong link, hope this works:

https://www.youtube.com/watch?v=I00HNlpo_Tg

Maybe my favorite Gloria.

https://www.youtube.com/watch?v=F5MHwHS5Doo
https://www.youtube.com/watch?v=lkYajAfjM3A

@Chaswjd, my band sang "Salvation is Created" a few times. It is wonderful!

I found this piece for a Thanksgiving service my choir was involved in last month.

Elgar, "Fear not, O land":
https://www.youtube.com/watch?v=AoNAhdRJsC4

Here's one of my favorite Ave Maria settings, Elgar again:
https://www.youtube.com/watch?v=GHOUfo1doqs

Walton's Jubilate Deo is a great sing:
https://www.youtube.com/watch?v=vlYWaFQ0qFI

William Billings "Jargon," while not sacred, is in a distinctly neoclassical idiom... it just happens to have been written in the 18th century:
https://www.youtube.com/watch?v=gtGh66mg7YQ

I love Charpentier's setting of Sub Tuum Praesidium. Listen to that cluster at the beginning!

Erm… how do I get this to work?