Agates, Carats and other Wisdom
By Theo Steinhauer
Theo Index
Photo
by
Jon Harman
Agates and Jaspers...Best Choice for Beginners
For cutting cabs, my favorite kinds of rough are certain colorful agates and jaspers that are hard and easy to polish and generally have few fractures, pockets and other imperfections. The agates that generally fit these specifications are Brazilian agate, Montana agate and many kinds of dendritic agate from US locations.
The jaspers that I like and seem to me take the best finishes are Bruneau jasper, Biggs jasper, Owyee jasper, many similar jaspers from US locations, the Imperial jaspers from Mexico, and bloodstone and fancy colored jaspers from India.
Moss and plume agates, many sagenite agates and jaspers and agates that merge into quartz formations are only second choices when choosing easy-to-cut rough. All of these stones require the touch of the experienced lapidary to deal with all the vugs, fractures, varying hardness and other problems likely to arise during the shaping and polishing of these stones.
Trumming
This is an ancient method for sanding and polishing delicate or intricate areas too small for hand or machine working. A small nylon cord is held in a vise. The other end is held in the hand, pulled tight and rubbed with abrasive or polish compound. The cord is then pulled through the opening in the design, pulled tight. Polishing and sanding is done by moving the work back and forth with pressure. Using tripoli on the cord does a fine job on sawed designs in silver or gold.
How Big is a Carat?
Cut precious stones are sold by the carat, along with the rough they are fashioned from and a carat is standardized as one-fifth of a gram and the carat is subdivided into a hundred parts called points.
The carat weights of some common objects are as follows: a common paper clip weighs 2.9 carats, abbreviated ct, a regular metal staple, 17 points, abbreviated pts, a dime, 11.25 ct, and a one cent, 15.55 ct. A brilliant cut one-carat diamond measures about 6.5 millimeters, mm, across the girdle.
Changing Color in Stones by Heating
Heat treatments of lapidary and jewelry rough stones have been practiced in the east for centuries. In Europe heat treating has been used for at least 200 years. Careful heating by gradually raising the temperature to the neighborhood of 600 degrees F and then cooling back to normal temperature alters the colors of some stones by changing the chemical structure of the coloring material. Both rough and cut stones are sometimes subject to this process.
Generally the stone becomes paler in color or it assumes a more attractive color. In several stones the color becomes darker. Some aquamarines lose their greenish cast and become darker in the process.
Many topazes are naturally an unattractive brown and heating often causes them to turn pink--all pink topazes have been heat treated. Zircons also and are all blue, colorless and golden-brown stones can be assumed to have been given this cosmetic lift. Amethyst becomes citrine by this method.