The LDS Edition KJV has a footnote on Gen. 6:16 that indicates some
Rabbinical commentators thought the "window" or "skylight" (Heb:tsohar)
was in fact some kind of glowing gemstone.  But, no references are
given.  Below are two Rabbinical sources that verify the statement.
The first is by "Rashi", or Rabbi Schlomo Yitzak, a very well-respected
11th century Jewish commentator.  The second is from a compilation
work for a Judaic Studies class at Brown Univeristy.

From _The Metsudah Chumash/Rashi: a New Interlinear Translation_, 1991.
Davis, Kornfeld and Walzer., KTAV Publishing House, 700 Jefferson St.,
Hoboken, NJ07030. Page 69.

  Gen. 6:16
  Make a skylight for the ark,
  and finish it to one *amoh* at the top.
  Place the opening of the ark in its side.

  On "a skylight" Rashi says:

  Some say this is a window.  Others say it was a precious stone that
  supplied them with light.


From _Genesis Rabbah, The Judaic Commentary to the Book of Genesis, A
New Translation_, Vol. 1, Neuser., Brown University Judaic Studies 104.

  [The formatting is unusual, but I'll preserve it as it appears]

  XXXI:XI
  1. A. "Make a light for the ark" (Gen. 6:16):
     B. R. Hunia and R. Phineas, T. Hanan and R. Hoshiah did not explain
    the matter
     C. R. Abba bar Kahana and R. Levi explained it.
     D. R. Abba bar Kahanna said, "It was a window."
     E. R. Levi said, "It was a precious stone."

  2. A. R. Phineas in the name of R. Levi: "During the entire twelve months
    in which Noah was in the ark, he had no need for the light of the sun by
    day nor for the light of the moon by night.
     B. "Rather, he had a precious stone, which he suspended.  When the
    stone dimmed, he knew that it was day, and when it glowed brightly, he
    knew that it was night."