text: Wow! 27 pounds is a very large size for a quartz crystal.  I would imagine if it really were quartz that it may be quite valuable.  Since quartz consists mainly of silica (SiO2), an unpolished stone may appear white on the inside.  In fact, sandstone is also composed of silica and since you mention that your stone is rusty yellow, without actually seeing it, I might suggest that it is a variety of sandstone.  There is also a chance that it may be citrine.  Please check out the links below.  There is a photo of rutilated quartz in the third link that sounds similar to what you describe.  If your stone is quartz, it would be great to try to sell it.  Good luck!