What is Porcelain white caps or bars in opals?

When opal is formed we sometimes see bars of white potch on the top colour bar or on the top of the potch. It is rare to find this white colour and it does seem to form differently in many opal fields.

BLACK OPAL

In lightning ridge the home of the black opal, some rough is found with white porcelain formed on top of the opal colour. In the Cochran opal field this has been considered a sign that the opal underneath this white bar with be brilliant fire colours, as these opals are formed in nobbys, that is like rocks or stone formation compared to seam opal found out at the sheepyards opal fields.

In seam opal you might see few thin lines white potch but has little or no effect on the opal colour compared to nobbys at the Cochran opal fields. In the Cochran opal fields the opal with white potch or white porcelain can be very hard actually harder than the opal.

Our cutting black opal specimen did have porcelain white top and we knew it would show good colour and it did show and electric blue opal. Another Cochran opal we cut had green veins and bottom of the opal was yellow grey potch base but the opal had white porcelain top on it

So when I cut this opal the noise on the diamond grit wheel sounded like I was rubbing metal against metal. Most opal is 5.5 to 6.5 on Mohs scale and I have cut Emerald and that is 8 on the Mohs scale and im sure this opal was way tougher than the emerald.

A steel file is only 6.5 on Mohs scale.

White porcelain is not to be confused with shin cracker. Shin cracker is formed close to opal colour so it is a good sign for the opal miner but it is very tough and chips off splinters and is called shin cracker as it hurts the shins and little cuts bleed.

It is more porous in structure than porcelain so porcelain must be formed more closer structure as it gives really good polish also.

Conclusion white porcelain tops in rough black Nobbys is good indication that the opal will be strong fire.but in ridge seam opal it seems to have little effect on opal colour

BOULDER OPAL

Boulder opal has been known to have more caramel potch in its formation more than white or porcelain. As boulder opal is formed in Ironstone rich mineral areas that most of the potch is brown colour from chocolate brown to tan colours.

Ironstone is a sedimentary rock and can consist either of oxides, i.e. limonite, hematite, and magnetite, silicates, i.e. chamosite or some combination of these minerals. So it is rare to find white porcelain ironstone due to the nature of ironstone formation.

But every now and then you will find small pocket of parcel boulder opal. This opal seems to have vegetation matter that is ironstone brown in colour and can display some opal colour

Conclusion

Carbonate is white colour and it must be this substance that makes this white porcelain colour or oin clay soils it could be Kaolinite that makes this white procelain. Shin cracker at lighting ridge is like porous carbonate with silica but it must be carbonate with dense silica that forms this beautiful porcelain white colour. So it must have to do with the pressure also when formed as not all porcelain white is dense and therefore makes a good polish. Even some Ethiopian opals will have bands of this white porcelain but only in veins

Even jasper is known to also has porcelain potch