help blx

I'm trying to make a polygon shaped as a diamond. But when I compile it's a triangle. Could anybody tell me what I'm doing wrong?

	glBegin(GL_POLYGON);							
        glVertex3f(-1.0f, 1.0f, 0.0f);
        glVertex3f( 0.0f, 2.0f, 0.0f);
        glVertex3f( 1.0f, 1.0f, 0.0f);
        glVertex3f(-1.0f, 1.0f, 0.0f);
        glVertex3f( 0.0f, 2.0f, 0.0f);
        glVertex3f( 1.0f, 1.0f, 0.0f);
	glEnd();

Here's how I think: http://img414.imageshack.us/img414/7650/namnls9fc.jpg yea i sux at glvertex3f -.-