l.cutler is correct. Toning. The number and type of tones and colors a clad coin can take on due to environmental exposure are numerous. I have handles samples of cupronickel clad coins that have toned in various shades of red, brown, green, and even a few blues. While such toning is unusual, it is not considered an error due to it having been manufactured to established standards.
As unlikely as it may be, if this coin is .990 copper, it will wear far more quickly than another quarter made with a clad planchette. Also, it would be a bit light. I have not, and will not, do the arithmetic for you.
Time for more coffee.
Edit: There are a few more possibilities. Commercial plating. Ads for gold plated quarters were quite common 15-20 years ago. I have no memory in my stroke addled brain of ads for copper plated quarters, but they may be out there.
Grade school science project. Copper plating in the classroom is easy and demonstrates many chemical and metallurgical principles.
Both of the above are more likely than a solid copper planchette, but less likely than toning.
Coins missing clad layers on both sides are extremely rare. I can see from the pictures that there is no weakness in the design elements, which would be very easy to see even if just one side is missing clad layer.
If the coin truly weighs only 2.3 grams, then it was either struck on a split Planchet (i.e. separated half of a normal Planchet), Foreign Coin Planchet or Experimental Planchet.
If it were missing the outer layers the details wouldn't be as sharp as they appear to be. Did you weigh another quarter to compare weights? Please take a pic of the edge of the coin.