These are natural opal gems, often left in their rough/natural state.  They showcase how the miners find them, when they are unearthed for the first time, after hundreds of millions, of years laying dormant.

They are not normally suitable for cutting or processing any further.  This may be due to a number of reasons or it may be simply that the value lies, in its natural form.  

Some collectors like the host rock, being part of the specimen and this also validates its authenticity.  Sometimes a small section may be "clipped" or slightly "rubbed" or polished to showcase the colour inside. A great investment if good colour is present and a great store of value, for wealth protection.  Not to mention, they are marvellous to look at and capture the hearts of hobbyists, collectors and investors alike.