Closed Back: Setting in which the pavilion of the stone is completely enclosed. May be foiled. Do not immerse vintage jewellery with closed-back settings in any liquid. This will very likely damage the appearance.
CLOSED BACK SETTING when a stone is set with no hole in the metal below it to allow light. Unless a gemstone is set in a tight bezel, cleaning under stones set this way is difficult.
The technical ability to mount gems with an open back had evolved from the earlier closed back mounting style, but it was not often seen in Georgian jewelry.
Foil backing - A method of color enhancement for colored stones that is rarely practiced anymore. Care should be taken when shopping for antique rings or jewelry with a closed back setting.
Doublets are easily spotted when the back is visible. If the opal is set in a closed back mounting, you cannot tell if it is truly a black opal or a doublet or composite stone.
To protect it, the back is filled in with resin. Such stones are normally bezel set with a closed backing to hide the fraud.
Top left: Round gas bubbles are visible in the resin backing under magnification.
Photos © Richard W. Hughes ...
Some are spheres and used like other round pearls; some are buttons and used to accent jewelry; and some are blister shapes called Mabé which are often used in earrings and other pieces with closed backs.
Prong mounted gems can usually be examined adequately as is, but bezel set stones (especially those with completely closed backs) often must be dismounted if their identity is to be conclusively verified.
A small piece of colored foil is applied either to the stone or to the closed back mounting. French Jet: Black glass that is often used to imitate jet.
 See also: Gemstone, Jewel, Jewelry, Gemstones, Stone
  
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