## Common ground
All options refer to cultured (synthetic) crystals used in industrial, scientific, or optical applications.
## Options at a glance
**2501.00 – <2.5 g, NaCl**
- **Defining trait:** Made of sodium chloride and each crystal weighs less than 2.5 grams.
- **Typical examples:** Lab-grade salt crystals for chemical testing or seeding.
- **Best fit when:** Small, non-optical NaCl crystals used in research or industrial processes.
**3104.20 – <2.5 g, KCl**
- **Defining trait:** Made of potassium chloride and each crystal weighs less than 2.5 grams.
- **Typical examples:** Calibration standards or reagents in analytical chemistry.
- **Best fit when:** Small KCl crystals for non-optical, non-structural uses like lab calibration.
**3824.99 – ≥2.5 g, MgO or alkali/alkaline-earth halides**
- **Defining trait:** Larger than 2.5 g each and made of magnesium oxide or specific halides (e.g., LiF, NaCl, KBr).
- **Typical examples:** High-purity crystals for radiation shielding, infrared optics, or industrial substrates.
- **Best fit when:** Large, non-optical crystals used in high-performance materials or engineering applications.
**9001.90 – Optical elements**
- **Defining trait:** Used as optical components (lenses, prisms, windows) despite being cultured.
- **Typical examples:** Infrared windows for lasers, crystal lenses in spectrometers.
- **Best fit when:** The crystal is shaped and used specifically for transmitting or manipulating light.
## Key differences
| Feature | 2501.00 | 3104.20 | 3824.99 | 9001.90 |
|---------------------|--------|--------|---------------|-----------------|
| Material | NaCl | KCl | MgO or halides| Any cultured crystal |
| Size | <2.5 g | <2.5 g | ≥2.5 g | Varies |
| Primary use | Non-optical | Non-optical | Non-optical | Optical function |
## How to choose
- Is the crystal made of sodium chloride and weighs less than 2.5 g? → 2501.00
- Is it potassium chloride and under 2.5 g? → 3104.20
- Is it larger (≥2.5 g) and made of MgO or a halide (e.g., KBr, LiF)? → 3824.99
- Is it shaped for optical use (e.g., lens, window)? → 9001.90
## Overlaps & pitfalls
A large NaCl crystal (≥2.5 g) could fit both 3824.99 and 9001.90 if used optically—priority should be based on **main function**. If it’s for optics, use 9001.90; otherwise, 3824.99. The term “optical elements” is clear but may mislead if the crystal is large and non-optical. Always confirm intended use