Answer:
   
   In his analysis of the meaning of the mitzvah of tzitzis (tassles
   placed on the corners of a four cornered garment), and in particular
   the thread of blue that one is supposed to place around it, R' Samson
   Refa'el Hirsch (Frankfurt-am-Main, Germany, 19th cent) writes (in
   Collected Writings vol III pg. 126):
   
     We find only three terms to encompass the colors of the spectrum:
     adom for red, yaroq for yellow and green, and techeiles for blue
     and violet...
     
     Red is the least refracted ray; it is the closest to the unbroken
     ray of light that is directly absorbed by matter. Red is light in
     its first fusion with the terrestrial element: adom, related to
     adamah [footstool, earth as man's footstool]. Is this not again
     man, the image of G-d as reflected in physical, earthly matter:
     "vatichsareihu me'at mi'Elokim" (Tehillim. 8,6)?
     
     The next part of the spectrum is yellow-green: yaroq.
     
     Blue-violet is at the end of the spectrum: techeiles.
     
     The spectrum visible to our eye ends with the violet ray,
     techeiles, but additional magnitudes of light radiate unseen beyond
     the visible spectrum. Likewise, the blue expanse of the sky forms
     the end of the earth that is visible to us. And so techeiles is
     simply the bridge that leads thinking man from the visible,
     physical sphere of the terrestrial world, into the unseen sphere of
     heaven beyond...
     
     Techeiles is the basic color of the sanctuary and of the High
     Priest's vestments; the color blue-violet representing heaven and
     the things of heaven that were revealed to Israel... no other color
     was as appropriate as techeiles to signify G-d's special
     relationship with Israel. A thread of techeiles color on our
     garments conferred upon all of us the insignia of our high-priestly
     calling, proclaiming all of us: "Anshei qodesh tihyun li--And you
     shall be holy men to Me" (Ex. 19, 6).
     
     If we now turn our attention to the pisil techeiles [blue thread]
     on our tzitzith, we will not that it was precisely this thread of
     techeiles color that formed the krichos [windings], the gidil
     [cord], the thread wound around the other threads to make a cord.
     In other words, the vocation of the Jew, the Jewish awareness
     awakened by the Sanctuary, that power which is to prevail within
     us, must act to unite all our kindred forces within the bond of the
     Sanctuary of G-d's law.
     
   The Talmud's desciption of the blue woolen thread reads: "The blue
   wool resembles the ocean, the ocean resembles the color of the sky,
   the sky resembles the purity of the sapphire, and the sapphire
   resembles the throne of G-d." (Chullin 89).
   
   Along similar lines, Israel's leaders get a vision of G-d on His
   Throne during the revelation at Sinai. The throne room is seen as
   being paved with "sapphire brick, like the essence of a clear sky."
   (Exodus 24:10) And the Midrash writes that the two tablets themselves
   were sapphire.
   
   Issacar, a tribe that was known for studying Torah full time, had a
   standard with a picture of a donkey on it on a field of sapphire blue.