The density of a gem is the mass per unit volume, and the unit is g/cm3.

The relative density of gemstones (formerly called specific gravity) refers to the ratio of the density of a substance (gemstone) to the density of pure water at 1 atmospheric pressure at 4℃, because the minimum density of pure water at 1 atmospheric pressure at 4℃ is the largest. Because it is a ratio (relative value), there is no unit. Density or relative density is very useful for identifying gems, because the same gem has its own relatively fixed density range. For example, the density of rubies and sapphires is between 3.95 and 4.05 g/cm3, and the density of diamonds is about 3.52 g/cm3. The density of cubic zirconia YAG, GGG and strontium titanate is much higher than that of diamond. Of course, some gems have the same density range as others. For example, some density values of ruby and garnet overlap, so it is difficult to distinguish them by density alone. So density is not the only basis for identifying gems. In addition, the inclusions in gems will also affect the density, so we must pay close attention to them.

Second, the method of measuring density and relative density

There are many methods for measuring density and relative density, which are briefly introduced below.

(1) Determination of gem density by hydrostatic method

Just weigh the quality of the gem, and then measure the volume of the gem, and divide the two, which is the density. The calculation formula is: density = mass/volume.

1. Determination method of mass (with balance)

(1) instrument balance, electronic balance with accuracy above 1mg or 0. 1mg. This balance is easy to operate (figure 12-9- 1).

(2) Operation steps of quality measurement

1) balance installation: Generally, the balance comes with it, so long as the vibration is small, there will be no problem. After installation, just put the weighing tray on it.

2) Adjustment of the balance: ① The balance must be installed on a stable table that is not easy to shake. The table should have no elastic vibration and its four legs should be stable. Generally, the table is a thick marble slab, neither too big nor too small; ② Adjust the horizontal state of the balance: there is a horizontal bubble inside the balance, which can be adjusted with two legs of the balance (which can be lifted and lowered by screws) to center the horizontal bubble and make the balance in a horizontal position; ③ Balance zero calibration: turn on the power switch, adjust the reading of the balance to zero, and put the standard weight into the scale; Read the mass of the standard weight (the standard weight must be calibrated by measurement, otherwise it will be invalid) and then correct it from the calibration hole of the balance; (4) The balance must be calibrated by the calibration unit before use, otherwise the data will be invalid. It is also necessary to correct the magnitude before daily measurement.

3) Weighing: ① Adjust the zero point; (2) Clamp the gem with tweezers and put it on the weighing plate; (3) Close the door of the balance, so as to avoid air flow affecting weighing; ④ Read out the quality data and record it.

Figure 12-9- 1 balance

2. Volumetric determination method

According to Archimedes principle, the mass lost by an object in water is equal to the mass of the same volume of water displaced by the object. In other words, the mass that an object loses in water is its volume.

Weigh the gem in water.

(1) Necessary tools

1) is enough to cross the bridge (small bench) and a cup, as shown in figure 12-9-2 ~ 12-9-4.

Figure 12-9-2 Schematic diagram of the device for measuring volume with a balance.

Figure 12-9-3 Weighing the quality of gems in water.

2) Special weighing frame with a certain height.

3) A small weighing basket or hook for holding precious stones.

(2) First, put a special weighing frame on the weighing plate. The weighing frame should be made of aluminum alloy with light weight and good rigidity, and the bottom area should be large to ensure stability. It can be a single arm or two arms with a hook at the top.

(3) Cross the weighing frame, place a small bridge, put a large-diameter water cup on the small bridge, and inject a certain height of water. Bridges and cups should not be in contact with the scale.

(4) Hang a hook or a small wire basket for putting gems on the weighing frame, and the height must ensure that all gems are immersed in water.

(5) Press the "Peeling Key" to make the reading zero (before putting the gem).

(6) Carefully hang the gem on the hook or put it in the basket.

(7) Read the weight of the gem in the water and record it.

Figure 12-9-4 shows the quality chart of gems in water.

Calculated density

(1) Calculate two readings.

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D—— gem density;

W1-the quality of gems in the air;

W2—— Quality of gems in water.

(2) If other liquids are used for determination, the following formula must be applied:

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D—— gem density;

W1-the quality of gems in the air;

W2-quality of gems in water;

D—— density of the liquid used.

4. Matters needing attention in determination

(1) The diameter of the liquid container should be larger. In principle, the larger the container, the better. The smaller the diameter of the container, the greater the liquid level rise, which will reduce the weight of the weighing frame, which is equivalent to increasing the volume of the gem and the greater the density error; Large diameter, small liquid level rise and small density error.

(2) The wires used to make the hooks and baskets should be thin to reduce the volume change caused by the rising liquid level.

(3) When weighing in other liquids, there shall be no bubbles on the gem. In addition, the measuring liquid adopts carbon tetrachloride with strong wetting ability and permeability, and the density accuracy is higher after carbon tetrachloride density correction.

(2) Density determination of large gemstones (hydrostatic method)

1. tool

(1) An electronic platform scale with large weighing capacity can be used, and its accuracy can reach 1g or 10g, and its weighing capacity can reach more than 35kg or 50kg.

(2) Large-capacity buckets or rectangular sinks.

(3) a thin and strong rope that does not absorb water.

2. Determination method

(1) Put the platform scale on the floor and adjust the level bubble to the center.

(2) Turn on the power supply and turn on the power switch. Wait for the bench scale to show zero, and use a 2kg weight for correction.

(3) Weigh the bucket with enough water on the platform scale, and record its quality data. If the volume is not large, you can peel it by pressing the peeling button, so that the scale shows zero.

(4) Tie the gem with a rope, leaving a piece of rope for lifting or hanging.

(5) Hang the bound gems in a bucket with a rope and immerse them below the water surface. Gemstones should not be exposed to the water or touch the bottom of the barrel wall, as shown in figure 12-9-5.

(6) Read the reading on the display and record (W2).

(7) Read the data on the display again after the gem has completely sunk, and record it (W 1). You can also weigh the gem first, and then put the bucket on the platform scale, which also represents W 1

(8) Density calculation

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D—— the density of the gem;

W1-the quality of gems in the air;

W2—— the mass of the gemstone lost in water (i.e. the volume of the gemstone).

(9) The relative error of this method is very small due to the large mass of the gemstone, and the volume of the rope will not affect the determination result.

The measurement principle of (10) volume is also based on Archimedes principle.

Figure 12-9-5 Schematic diagram of gems hanging in barrels with ropes.

(3) Estimate the relative density (range) of gemstones by heavy liquid method.

1. principle

The density of heavy liquid is known, and the relative density of precious stones (solids) can be judged by their suspended state in the liquid. Generally, there are the following three situations:

(1) If the density of a solid is less than that of a liquid, the bigger the difference between the two, the faster and higher the floating speed, and the solid will float on the liquid.

(2) If the density of a solid is lower than that of a liquid, the solid will sink to the bottom of the liquid. The greater the difference between the two densities, the faster the decline.

(3) If the density of a solid is close to that of a liquid, the solid will be suspended in the liquid. If it is slightly higher than the density of the liquid, it will slow down and finally sink to the bottom. If it is slightly lower than the density of the liquid, it will slowly rise until it is exposed to the surface of the liquid.

Step 2 prepare a series of heavy liquids

It is necessary to prepare a series of heavy liquids to determine the relative density. Their density values are 2.57, 2.62, 2.67, 3.05 and 3.32g/cm3 respectively, and the density of saturated saline is 1. 13g/cm3.

The raw material for preparing heavy liquid is organic solution, which requires (1) stable chemical properties and does not react with precious stones; ② Non-volatile; (3) non-toxic or low-toxic, and does not pollute the environment; (4) easy to clean, no residue. Generally, the following organic raw materials are used:

Chemical formula of raw material density

Diiodomethane 3.32g/cm3

Tribromomethane 2.88g/cm3

Chloroform1.49g/cm3 chloroform

α-bromonaphthalene1.48g/cm3 C 10H7Br

Cleric solution 4.20g/cm3 dichloromethane (COO) NH2O.

Adding benzene, xylene, toluene, vinyl bromide or carbon tetrachloride in different proportions to diiodomethane, a series of heavy liquids with densities of 3.32 ~ 2.57 g/cm3 can be prepared.

Diiodomethane and tribromomethane can be prepared into heavy liquid with density of 3.32 ~ 2.88 g/cm3.

Tribromomethane and bromonaphthalene can be prepared into a heavy liquid with a density of 2.88 ~ 2.57 g/cm3. A series of heavy liquids with density of 4.20 ~ 1.00 g/cm3 can be prepared by adding water to the priest's liquid. But the priest's liquid is made of thallium acetate, which is very toxic and can penetrate into the human body through the skin. In addition, this kind of liquid is very expensive, and the waste liquid pollutes the environment seriously, so it is generally not used. Put the prepared heavy liquid into a bottle of the same size, and close the lid for later use. Bottles are generally 20 ~ 25mm in diameter and 50 ~ 60mm in height (Figure 12-9-6).

Figure 12-9-6 State of Gemstones in Heavy Liquid

3. Determination method steps

(1) tool

1) a cup of cleaning solution, usually xylene, ether and alcohol or alcohol.

2) A pair of tweezers (for holding gems)

3) A series of heavy liquids with different densities, in which solids control the density of heavy liquids.

(2) Operating steps

1) Wash the gems with cleaning solution and dry them.

2) Hold the gem with gem tweezers and put it into the selected heavy liquid. (weigh it by hand in advance, estimate the density, and then choose the heavy liquid), as shown in figure 12-9-6.

3) Loosen the tweezers and observe the suspended state, settlement or rising speed of the gemstone in the heavy liquid.

4) Select heavy liquid: ① If the gem rises, it should be tested with another heavy liquid with lower density; (2) If the gem rises rapidly, it can be tested with a heavier liquid with a smaller density; (3) If the gem falls, it should be replaced with heavy liquid with higher density and re-tested; (4) If the gem drops rapidly, it should be tested with heavier liquid; ⑤ If the gem is suspended in the middle of heavy liquid, the density of the two is very similar; ⑥ If the gem is suspended in the middle of the heavy liquid and rises slowly, it means that the density of the gem is slightly less than that of the heavy liquid; ⑦ If the gem is suspended in the middle of the heavy liquid and slowly descends, it means that the density of the gem is slightly higher than that of the heavy liquid.

All of the above can be recorded. If you can't suspend, you can only measure the density of the gem between them, and then record the density range of the gem.

Note: ① Every time it is taken out from the heavy liquid, it must be washed with cleaning solution, dried and then put into the second heavy liquid. Otherwise, heavy liquid A will be brought into heavy liquid B, and the density of heavy liquid B will change. (2) Solid with standard density must be put into the heavy liquid to prevent the density change of the heavy liquid from being discovered as early as possible.

5) Matters needing attention in measuring gem density by using heavy liquid: ① The heavy liquid must be calibrated (pycnometer method); ② There are many factors affecting the density of gemstones, especially the inclusions in gemstones; (3) When the refractive index of the gem is close to the heavy liquid, the protrusion is very low, and the position of the gem is difficult to determine; (4) when replacing heavy liquid, samples and tweezers should be cleaned and wiped clean; ⑤ Put only one sample at a time to prevent mixed samples; ⑥ The heavy liquid should be stored in a low temperature and dark place, and a piece of copper should be put in the heavy liquid containing iodine to prevent the diiodomethane from decomposing into iodine, which makes the liquid black and impossible to observe; ⑦ Temperature will affect the density of heavy liquid. When the temperature rises, the density of heavy liquid will decrease, so it is necessary to determine the density of heavy liquid in time. ⑧ Heavy liquid vapor is toxic, so avoid inhaling into human body.