Carat and Size of Gems
Weighing Gems:
In the early history of gem marketing, depending on the geographic location, one of two common items, familiar to both buyers and sellers, was used to measure the amount of gem material being bought and sold: the wheat grain and the carob 稻子seed. Each of these commodities was known for being particularly uniform in size and weight. We still see remnants of this early system in today's terms: "carat" the international metric unit used for gems, and "grain" a unit sometimes used in selling pearls, and also in today's system of apothecary 药剂师measure.
**As we work through this section, you'll probably begin to wonder why it's all so complicated, confusing and haphazard seeming. Unfortunately, the system in place today developed bit by bit from mergers 合并and splits amongst pre-existing local systems. The, sometimes frustrating, result is pretty much of a hodgepodge大杂烩. Although some degree of uniformity has been introduced by the use of the metric system, things still are far from predictable and totally logical.
Carat: The carat, pronounced like the vegetable, carrot, and abbreviated "ct" is 0.2 grams. So, there are five carats per gram. The metric system is the basic international standard used for gem commerce. Many of us who live in the US or UK where English measure is more common, need to take time, and do some practice, in order to get a "feel" for carats, grams, etc. The ounce, a familiar English unit of weight, equals approximately 142 cts. So, there really isn't an appropriately small unit in the English system which could be easily applied to gem weights. [To illustrate: a 1 ct. gem weighs 0.007 oz.]
Another oddity of the US system is our use of the term "karat", also pronounced like the vegetable carrot, but abbreviated "k" or "kt" to indicate the fineness (purity) of gold. In most other countries, the purity of gold is indicated by the number of parts of gold out of 1000, such as 585 or 750, so there is no chance of confusion with gem weights. The number 585 means that 585 out of 1000 parts of the alloy are gold or, in other words, that the gold content is 58.5%.
In comparison, the karat system uses the number of parts out of 24 that are gold. 24k means 24/24th, pure gold, also known as "fine" gold, 18k gold = 18/24th gold, and 14k = 14/24th gold. (14k and 18k and 24k translate then, in the International system, to 585, 750 and 999 respectively).
**Check the text: See Lyman, pg. 41. The authors (who are Italian and can be forgiven because as Europeans they don't use the karat system, mistakenly use the word "carat" instead of "karat" in describing how gold is marked in the US, and Lyman, the American editor, didn't catch it. So, don't you be confused: For gems it's carat, for gold it's karat !!
[Stamp on a 24k or "fine" gold piece]
The time honored way that jewelers and gold dealers tested gold purity was by using acids and a set of test needles of known karatage. A streak条纹 was made by the object being tested on a stone plate and comparison streaks made below it with the test needles. Then the acid solution was applied to all. Based on the degree and color of the reaction, compared to the test streak reactions, the composition could be closely approximated.
We get our terms "touchstone" and "acid test" from this ancient procedure. Kits工具箱 using this same principle are sold, and still widely used today, although a newer system based on electrical conductivity is becoming popular. In devices of this newer type the test object is immersed in a few drops of electrolyte solution, and then subjected to a current--> its purity level can then be read directly from the scale.
["Acid Test" kit, electronic gold tester: images courtesy of Prettyrock.com]
Getting to know the carat:
Below, you see the carat weights of three common objects: since you are likely to be familiar with their approximate weights, this can perhaps help you begin to get a "feel" for the weights represented in carats.
Common items weighed in carats:
[Small, (1.5" x 2.0") Post-it note = .75 ct., standard bobby pin = 2.8 ct., dime = 11 ct.]
Special Cases:
Pearls
Pearl Grain: The pearl grain, is .25 grams, so one gram is equal to 4 pearl grains. Thankfully, the only remaining use of this once important measure, is sometimes seen in the sale of natural pearls by weight. Because there is very little commerce today in natural pearls (virtually all pearls on the market are cultured), it is fast becoming obsolete.
Many cultured pearl wholesalers still sell bundles of pearls in larger units called "momme姆米" which, historically, weighed 75 pearl grains.
Cultured pearls are sold by diameter (millimeters) if they are round, or near round, and by carat if they are oddly shaped (baroque).
[9 mm. round cultured pearl, 8.4 ct. baroque cultured pearl]
Check the web: The folks at pearl-guide.com have (among articles on just aspect of pearls you'd like to learn about), a short and clear web about any page explaining pearl weights: http://www.pearl-guide.com/pearl-weights.shtml
Melee & Total Weight
Melee: Gems weighing .20 ct. or less are referred to by the gem trade as "melee". They are most often not sold by weight, but rather by girdle diameter: 2 mm., 3.5 mm., etc. Such stones are generally used as accents, for cluster settings, or in pave' work.
Total weight: When a jewelry piece has more than one stone, such as a center stone and accents, the total carat weight, must be used: abbreviated as "ct. tw."
[Ring with diamonds and Tsavorite pave' melee of .70 ct. tw., pendant with rubellite tourmaline and diamonds: .66 ct. tw.]
Big Items
Gem rough, and in some cases, carvings and ornamental objects are sold by the gram, (gr) or kilogram, (kg) as the carat is an inappropriately small unit for such goods. Occasionally, you see such wares with simply a per piece price without any weight measure listed at all.
[57.5 gr. ruby in zoisite gem carving ]
Metals
Metals, like gold, platinum and silver, are not weighed in the metric system of carats and grams, nor the English system of ounces and pounds, but in the "Troy金衡制盎司 " system. Unfortunately, the Troy system also uses the terms "ounce" and "pound" but these terms are not equivalent between systems.
When you hear that gold is selling at $900 per ounce, it is a Troy ounce which is about 10% heavier than an "English" ounce. (An English ounce = ~ 142 ct. whereas a Troy ounce = ~ 156 ct.) Troy ounces are subdivided into smaller units called pennyweights, abbreviated "dwt.". There are 20 dwt./ troy oz. Jewelers generally buy their gold casting grain, by the pennyweight. To further complicate matters, there are 12 Troy ounces in a Troy pound rather than 16 oz/lb as in the English system!
Remember, in the last web lecture, gold was said to be worth substantially less than $10.00/ct.? We can now see how that figure was calculated: gold at $900 per ounce (Troy) = $900 per 156/ct., so dividing 156 into $900 gives us $5.77/ct. Check the current price to get a more accurate figure.
Food for thought: Presuming you want the most gold possible: (Answers to the questions are found at the end of the lesson.)
Question One: Would you rather have a Troy ounce of gold or an English ounce?
Question Two: Would you rather have a Troy pound of gold or an English pound of gold?
Precision
In commerce, colored stones are generally weighed to 0.1 ct. and diamonds are usually weighed to .01 ct. Each 1/100th of a carat is called a "point". So, one could alternately describe a 0.50 ct. diamond as weighing 50 points. {Interestingly, in the world of diamond sales, 50 points is not precisely the same as "1/2 carat". Fractional parts of carats actually refer to ranges! It is legal and proper to advertize and sell to any diamond within the range of 0.45 ct to 0.55 ct. as a 1/2 carat stone.
Tools for Weighing
Long ago, gems and precious metals were weighed for trade by using simple hand held or platform mounted pan balances. The dealer placed the requisite number of carob seeds or wheat grains (common items with very standard weights) in one pan and added gems or gold in the other pan until the two pans hung level. Although this sounds primitive, a practiced user can get very accurate weights, and such tools are still in use in much of the world, although carob seeds have been replaced by tiny, carefully calibrated metal "weight standards" marked in carats or grams.
[Antique brass pan balances]
Several decades ago, mechanical spring balances or beam balances were state of the art, today, however, virtually all gems are weighed on electronic scales. The basic principle is the same as that of the spring or tension balance (like the kind you weigh produce in at the grocery store). The difference is that the pressure from the object being weighed, instead of stretching or compressing a spring, creates increased electrical resistance. The result is displayed digitally as the object's weight.
[Electronic carat scale]
Factors affecting weight:
It might seem, at first thought, that all 6 millimeter round gems would weigh about the same, but there are two important factors which greatly affect individual gem weight: 1) the density of the material (its weight per unit), and 2) the proportions of the cut. In the next lesson we'll learn more about gem density (specific gravity) but the basic idea is that some gem species weigh more per unit than others, just like a 4" cube of steel will weigh more than a 4" cube of oak. (Sapphire, for example, has a higher density than quartz, so a 6 millimeter round sapphire, all other factors being equal, would weigh more than a 6 millimeter round quartz.)
The cut, particularly in regards to the pavilion depth and degree of pavilion bulge膨胀, is equally important in determining the weight of any given gem of a certain length and width. The diagram below shows two gems of the same face up dimensions, let's say 6 millimeter rounds, but which are cut to very different proportions. The deep or "belly" cut gem weighs much more, both due to the greater depth of the pavilion and to the bulging out of the sides. It is quite common to find "native cut" gems of this type. This is partly because the lapidaries in the country of origin are frequently paid by weight, but also because such gems, though awkward to mount in standard Western commercial settings, deepen the apparent color of lighter gem materials.
[The effect of cut on gem weight: Image courtesy of www.tripps.com]
Measuring Gems:
The common household ruler, generally has inches on one side and millimeters on the other side. It's a good item to keep handy when first attempting to get a feel for metric gem measurements. If you saw a description of a gem that says it measures 8 x 10 millimeters, that might not bring up an immediate mental picture of its size. Using your ruler, it's pretty easy to make a small sketch to represent the gem. Doing this a few times is all that's necessary to begin to think more easily in millimeter sizes.
Such a ruler isn't precise enough for jewelers or gem dealers, who have a variety of moderately to extremely accurate measuring devices at their disposal. The simplest, least expensive, and most portable of these is the engraved brass sliding pocket gauge, seen below. The gem is placed between the jaws which are gently slid into contact with it. The lower scale generally reads in millimeters and the upper scale in inches.
For greater precision there are several other options to choose from, such as the screw micrometer and the digital sliding gauge数字游标卡尺.
[Brass sliding gauge]
[Digital sliding gauge]
With the simple brass sliding gauge accuracy is to tenths of millimeters, hundredths of millimeters must be estimated, not so with the digital version.
A specialized type of jeweler's measurement tool, which has great versatility广泛用途, is called the Leveridge gauge宝石卡尺, available in both mechanical (seen below) and electronic types. Besides measuring loose gems, this device is useful in taking certain measurements on gems already set into jewelry. This can be done directly if the setting is open on the bottom--> one prong goes on the gem's table and the other on its culet or keel. In addition, the pointed jaws opposite the prongs, can be used to get pretty good estimates of distances, even if the setting is closed.
[Leveridge gauge: Image courtesy of www.riogrande.com]
Also favored by jewelers are stone and hole gauges, which can be used to get fairly accurate estimates of the dimensions of a given gem, or the size of a particular opening on a setting.
Where to Measure ?
Having a good tool is necessary, but you also need to know the appropriate place to take each measurement. Length and width are the two primary dimensions of interest, although in the formal cut grading system for gems a large group of other measurements are taken, such as table width, total depth, crown height, etc.
Each of the regular shapes of gemstones has a preferred position for taking length and width measures. Most are pretty obvious (the two longest perpendicular dimensions), but the special situation of the heart shape bears mentioning. The length of heart shaped gems is measured from a hypothetical line joining the tops of the lobes耳垂, rather than from the cleft裂缝.
[Correct measurement sites for common shapes: Graphic courtesy of www.tripps.com]
Answers to the thought exercises for this lesson. (If you don't understand why these are the correct answers, then it's a good time to email me and ask!)
1): Troy ounce. If we use carats, then it's easy to decide on the Troy ounce which weighs 156 ct. compared to the English ounce's 142 ct.
2): English pound. Since there are only 12 Troy ounces in a Troy pound, then there are only 12 x 156 or 1872 carats in a Troy pound, whereas there are 16 x 142 = 2272 carats in an English pound, as English pounds have 16 ounces each.