I have a few stones that I am unable to identify. Can you please help?
I can not give you a definitive answer to your question. It is very difficult to identify a rock from pictures, there are about 100,000 different rocks and minerals and to identify one or two, as you can imagine, would be very difficult. To identify rocks/minerals, other than the ordinary, there is a sequence of tests that one must commence i.e. color, hardness, streak, habit, crystal structure, malleability, blowpipe analysis, specific gravity or lab analysis. Other factors that have to be considered are location, formation location, location within a formation and structure of the formation.
I can however answer a portion of your question. Your specimen appears to be a rock with a secondary coating of opal. Occasionally, when conditions are ideal, spheres of silica, contained in silica-rich solutions in the earth form and settle under gravity in a void to form layers of silica spheres. The solution is believed to have a rate of deposition of approximately one centimetre thickness in five million years at a depth of forty metres. If the process allows spheres to reach uniform size, then precious opal commences to form. For precious opal the sphere size ranges from approximately 150 to 400 nanometres producing a play of colour by diffraction in the visible light range of 400 to 700 nanometres.
Each local opal field or occurrence must have contained voids or porosity of some sort to provide a site for opal deposition. In volcanic rocks and adjacent environments the opal appears to fill only vughs and cracks whereas in sedimentary rocks there are a variety of voids created by the weathering process. Leaching of carbonate from boulders, nodules, many different fossils, along with the existing cracks, open centres of ironstone nodules and horizontal seams provide a myriad of moulds ready for the deposition of secondary minerals such as opal.
Much of the opal deposition is not precious. It is called "potch" by the miners, or common opal by the mineralogist, as it does not show a play of colour. Opaline silica not only fills the larger voids mentioned but also may fill the pore space in silt and sand size sediments cementing the grains together forming unique deposits, known as matrix, opalised sandstone or "concrete" which is a more conglomeratic unit near the base of early Cretaceous sediments.
The many variations in the types of opal depends on a number of factors. In particular, the climate provides alternating wet and dry periods, creating a rising or more importantly a falling water table which concentrates any silica in solution. The silica itself is formed either by volcanic origin or by deep weathering of Cretaceous clay sediments producing both silica and white kaolin often seen associated with the Australian opal fields. Special conditions must also prevail to slow down a falling water table in order to provide the unique situation for the production of its own variety of opal.
The chemical conditions responsible for producing opal are still being researched, however some maintain that there must be acidic conditions at some stage during the process to form silica spheres, possibly created by microbes.