To test if a black diamond is real, you can use several methods. Here are a few reliable ones:

### 1. **Thermal Conductivity Test**
- **Diamond Tester**: A handheld device that measures thermal conductivity. Diamonds dissipate heat quickly, so if the diamond tester indicates high thermal conductivity, it's likely a real diamond.

### 2. **Scratch Test**
- **Hardness Test**: Use a piece of corundum (like ruby or sapphire) to scratch the surface of the diamond. Real diamonds are harder than corundum (diamonds rank 10 on the Mohs hardness scale), so they should not scratch.

### 3. **Magnification Test**
- **Loupe or Microscope**: Under 10x magnification, real diamonds often show small imperfections or inclusions. Lab-created diamonds and some other gemstones may appear flawless or have different types of inclusions.

### 4. **Refractive Index Test**
- **Fog Test**: Breathe on the diamond to fog it up. Real diamonds dissipate heat quickly and should clear up almost immediately, while fakes might stay foggy for a few seconds.

### 5. **Electrical Conductivity Test**
- Some diamonds (especially synthetic ones) conduct electricity. Devices that measure electrical conductivity can help distinguish between real and synthetic diamonds.

### 6. **UV Light Test**
- **Fluorescence**: Place the diamond under UV light. Some diamonds fluoresce (glow) under UV light, usually a blue color, which is an indication of a real diamond. However, not all diamonds fluoresce, and some fakes might as well.

### 7. **Weight and Density Test**
- **Hydrostatic Weighing**: Measure the density by comparing the diamond's weight in air versus its weight in water. Diamonds have a specific density, and deviations might indicate a different material.

### Professional Evaluation
If you're uncertain after these tests, it's best to take the diamond to a professional gemologist or a certified diamond grader who can use advanced equipment to determine its authenticity.

Always handle diamonds carefully during these tests to avoid any potential damage.

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