This is one of the first really tough concepts to get into your head when you begin with an object-oriented language like
Java. There's a fundamental difference between primitives (like int and double) and Classes, like your Jewel.
When you say:
Casa miCasa;
you have created something called a reference, which can refer to an object of type Casa. However it doesn't currently refer to anything. No Casa has been created. You can make it refer to an already existing Casa object by saying:
miCasa = tuCasa;
In this case, there is still only one Casa, but both miCasa and tuCasa refer to it. On the other hand, you can construct a new Casa and have miCasa refer to that.
miCasa = new Casa();
Now, miCasa and tuCasa refer to two different Casas.
String, incidentally, is a bit of a hybrid. You could treat it like any other Class, like so:
String hint = new String("Casa is Spanish for house");
but the Java language provides special String literals, so you can just say:
String myString = "I hope I'm not confusing you!";
Very few classes have convenient literals like this, and none of the ones you define do, so you'll have to write:
Jewel myJewel = new Jewel();
or something similar, depending on the constructors defined in the Jewel class.