Despite being the most common rock-forming minerals at the earth’s surface, the stones in the feldspar species give jewelry makers some of the most visually fascinating material available. The feldspars have gained tremendous popularity with jewelry buyers, and thanks to their dramatic optical effects, demand for these stones continues to remain high.
Feldspar is a very appealing species of stones. It provides a great choice of natural colours, comes in large sizes, is good value for money and has beautiful optical effects that you will never get bored with. The feldspars are often described as having a schiller, opalescence or adularescence. These terms all refer to the shimmering effect that appears to sit just below, or above, the surface of the stone when it is placed or moved under a light. Moonstone is well known for this special optical effect, in which reflected light causes a blue or white schiller.
This internal reflection of light, or “interference”, also produces an effect called iridescence; an example is the rainbow of colours in labradorite. The feldspar group is subdivided into two distinct varieties: the orthoclase (alkali) feldspars, which include moonstone, orthoclase and amazonite, and the plagioclase feldspars, including labradorite and aventurine feldspar.
Treatments and imitations
- Glass imitations of sunstone and blue moonstone exist.
- Yellow orthoclase could be confused with pale yellow citrine, topaz, prehnite or yellow beryl.
- Amazonite could be confused with green jade, chrysoprase or turquoise.
- Rainbow moonstone is much cheaper than blue moonstone. Good crystal-clear green moonstone is moderately priced; it is more expensive than rainbow moonstone but does not reach the price of clean blue moonstone.
- Sunstone is probably the most expensive form of feldspar. With Tiffany & Co. featuring this material in their jewelry range, it is possible that the stone will gain a popularity and demand similar to tanzanite’s.
Working with feldspar
- The layered structure of feldspars affects their durability as well as their optical properties. Internal layers and cracks will often break through to the polished surface of the stone; rainbow moonstone and labradorite are both particularly prone to this. The perfect cleavage means that a slight blow, or too much pressure, will break a stone in two, so care needs to be taken when setting, soldering and polishing jewelry. It is probably not a good idea to use feldspars for bracelets or cufflinks. It is also important to store and protect stones, beads and jewelry properly, as they can become chipped and scuffed very easily.
- Stones that have optical effects should be set in such a way that any schiller or iridescence is fully displayed; it is worth experimenting to find the best position. Stones such as labradorite need movement in order to catch the light and flash with colour, so rings and drop earrings are better than necklaces, which are more static. A vertically hung pendant won’t display a stone with a star as well as a ring on a hand that rests on the horizontal plane. Using a closed-back setting or painting the inside of the setting black will emphasize the blue schiller of a blue moonstone, displaying the sheen at its most intense. Any stars or cat’s- eyes should be set straight, so that the eye or star is in the centre of the stone. To emphasize the inner light of clear stones such as green moonstone, polish the inside of the setting and make sure that no visible dirt is trapped behind the stone.