White Ironstone: A Survey of Its Many Forms: Undecorated, Flow Blue, Mulberry, Copper Lustre (Schiffer Book for Collectors With Value Guide.) by Dawn Stoltzfus, Jeffrey Snyder; Paperback ...
Ironstone
A rock containing enough iron to permit commercial extraction; an iron ore.
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This ironstone rock is a natural mixture of hematite, jasper and tigereye. Tiger iron is a great example of a metamorphosed gemstone.
Boulder opal is found sparsely distributed over a wide area of Australian ironstone or boulder country where the opal (silica mix) fills veins, cracks, cavities and crevices in ironstone boulders.
When the opal is distributed throughout the ironstone matrix in a form not suitable for recovery, the material is often used for unusual decorator objects.
General opal classifications are: "Boulder Opal" which is opal on a dark ironstone matrix (typical occurrence: Quilpie, Queensland); "Crystal Opal" which is transparent to semi-transparent black, ...
Boulder opal forms on a dark ironstone base (the host rock) and occurs as a thin uneven layer adhering to the ironstone. Because of the uneven layers, sometimes part of the ironstone is visible on the surface of the stone.
Boulder Opal, formed in a sandstone matrix called ironstone, has light flashes of blue against a rich pattern of dark brown, reddish brown, and tan.
Boulder opal is found in Australia where precious opal forms in veins and patches within brown ironstone boulders. When the opal is mixed through the ironstone it is called matrix opal. Hardness ranges between 5.5 and 6 on the Mohs scale.
Boulder Opal - opal with play-of-color that is present in a matrix of dark brown or black sandstone (ironstone).
A good example is the opal triplet which often is an ironstone backing dressed with a slice of precious opal which on it's turn is covered by a crystal cabochon.
Boulder opal combines precious opal with the ironstone in which it forms. Bright yellow, orange, or red fire opal are quite different from the other varieties of opal.
Potch
Native material around a gemstone, like ironstone surrounding opal.
Precious Gem
The quality of a gem that has beauty, durability and rarity.
A form of Boulder Opal from Yowah (Queensland), Australia. It occurs most often as a nut-sized ironstone nodule containing pockets and veins of vivid Precious Opal.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z ...
A thin layer of opal is sandwiched between a layer of clear quartz and a layer of either obsidian or ironstone. The clear quartz is the top layer, making the gem harder (and less susceptible to scratches).
The first two are blue fibrous minerals, with glaucophane occurring in blueschists and crocidolite (blue asbestos) in ironstone formations, both resulting from dynamo-metamorphic processes.
This type of opal stones has a backing of an ironstone matrix which is natural to the stone and they are not doublets because the stones are one piece.
Opal sometimes forms as a thin layer in ironstone known as boulder opal. Cutters fashion it into free-form cut stones with the opal in front and the natural rock behind.
Potch...native material in or around a gemstone, a dead spot or area. ie: ironstone surrounding opal.
Radioactive...Emitting alpha, beta, or gamma rays.
It possess natural brown body colour or the back that appears black from the front. It is too available in all colour variations. Often it possesses rolling surface and ironstone inclusions. It can be given any innovative cut.
Australian opal ...
Associations:
Produced by hydrothermal alteration of limestone. May also occur as concretionary or oolitic forms in clay ironstones.
Opal doublets, for example, may use backings consisting of ironstone, black plastic, quartz, or whatever material brings out the best in the host. Doublets are made from a wide variety of natural, synthetic, and artificial materials.
Another more unusual type of opal is boulder opal, which has opal with an ironstone host rock matrix which creates a natural dark background to view its fire.
is made by sandwiching three thin layers of stones together. For example, an opal triplet had a top, protective layer of clear quartz, a thin middle layer of opal, and a base layer of dark, color-enhancing matrix (usually black onyx or ironstone).
 See also: Stone, Color, Jewel, Gemstone, Opal
  
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