How to use a Penlight for Identifying Gemstones

A penlight is useful for determining the quality of star gems and gems with special light effects. These are gems with chatoyancy (tiger eyes), asterism (star sapphires), adularescence (moonstone), iridescence (agates), and other plays of color such as opals and sunstone. Penlights are also good for seeing into cut and uncut gems for fractures, color, and quality.



Place the gem in a dark area out of direct light when checking for the quality of a star sapphire or ruby. Asterism should only be seen in light. For example, if you can clearly see a star effect in the dark then the stone is not real. Shine the pen light directly on the stone to reveal the special light effect. Shine the penlight at a gem from different angles when checking for fractures.



I use my penlight most frequently while visiting the gem dealers in the border town of Mae Sot, Thailand, directly across from Myawaddy, Myanmar. This is my favorite place to buy star sapphires, moonstones, and jade. For those new to the trade, while quality is desirable, sometimes a jeweler needs many medium or low quality gems to assemble a large necklace, broach, or pendant, where design is paramount. However, a quality jeweler wants to work with real gems, and low quality gems can be very desirable for use in different ornamentation.



Unfortunately, all gems can be synthesized and imitations abound. Even when you want to buy very inexpensive gems, imitations are so cheap to make that they are often presented as real. All qualities of imitation star sapphire and star rubies are used to cheat people. Luckily, some simple tests with a pen light can keep you from being fooled.



The first rule is simply, no light, no star. Put the gem in the dark, or cover it with your cupped hand. Do you still see a brilliant star? If you can, do not buy it. Next, shine the pen light on the gem, do you see the star clearly, that’s a good sign of quality. Note where you see the star, is it in the center or off to a side? Poorly cut star gems will have the stars off to one side. Stars may also be off center because of its location on the gem made it impossible to cut it otherwise. All the same, this is not desirable for rings and pendants where the gem is clearly on display.



Penlights are also very useful for semi-opaque gems, such as cabochon cut rubies, sapphires, and jades. Shine the light through the bottom of the gem to check for fractures. Often time’s tourists purchase cabochons with deep fractures which split the moment the gem is tapped to hard. For jade, look to see that the material has the same color gradient throughout the stone. Many jades are manufacturers by laminating a good piece of jade to low quality jade. Did you notice I said to look for a color gradient? Jade is often dyed, as are many other soft stones to look like high quality jade. These jades will have uniform color, or sometimes the outside will be much greener than the inside, with an obvious boundary layer.



Moonstones are going to appear to glow from inside. In Mae Sot, they will be milky, translucent, and give off a soft glow when seen in light. They are often imitated with resins. Like most real gems, they will never be too perfect.



For more information about buying gems in Thailand, get our book, Gem Buyers Guide to Thailand , available on Amazon.com. Come and visit us at PGS Jewelry on Nimmanhaemin Road in Chiang Mai, or visit our web site at www.pgsjewelry.com.