## Common ground
All options refer to magnesium oxide (MgO) in different forms and applications.
## Options at a glance
**2519.90 – Natural magnesium oxide**
- **Defining trait:** Unprocessed or minimally processed MgO from natural sources, not refined into crystals or optical components.
- **Typical examples:** Magnesium oxide powder from mineral deposits, raw feedstock for industrial use.
- **Best fit when:** The product is bulk, non-crystalline, and used in chemical, agricultural, or industrial processes.
**3824.99 – Cultured crystals (other than optical elements)**
- **Defining trait:** Synthetic, grown crystals of MgO (or similar salts) ≥2.5 g each, not intended for optics.
- **Typical examples:** High-purity MgO crystals used in research or as substrates in materials science.
- **Best fit when:** The product is a large synthetic crystal, not used in optical devices.
**9001.90 – Optical elements (not permanently mounted)**
- **Defining trait:** Transparent MgO components shaped for light manipulation, not yet installed in instruments.
- **Typical examples:** Unmounted lenses or prisms made of MgO for infrared optics.
- **Best fit when:** The product is a cut, polished optical component used in scientific or industrial optical systems.
**9002.90 – Mounted optical elements**
- **Defining trait:** MgO optical components permanently installed as parts of instruments or apparatus.
- **Typical examples:** Infrared detectors with built-in MgO windows or lenses in spectrometers.
- **Best fit when:** The product is integrated into a device and used for measurement, sensing, or imaging.
## Key differences
| Option | Form | Size/Shape | Mounting | Primary Use |
|-------|------|------------|----------|-------------|
| 2519.90 | Natural powder | N/A | N/A | Industrial/chemical |
| 3824.99 | Large synthetic crystal | ≥2.5 g | Not mounted | Research, substrates |
| 9001.90 | Polished optical element | Any size | Not mounted | Optical systems |
| 9002.90 | Mounted optical component | Any size | Permanently mounted | Instruments |
## How to choose
- Is it raw or natural material? → 2519.90
- Is it a large synthetic crystal (≥2.5 g) not used for optics? → 3824.99
- Is it a polished optical piece not yet installed in a device? → 9001.90
- Is it part of or fitted into an instrument? → 9002.90
## Overlaps & pitfalls
- A large cultured crystal (3824.99) could be used as an optical element, but only if not mounted—otherwise, it falls under 9002.90.
- If a product is both a crystal and optical, the **main function** determines the code: use 3824.99 for non-optical crystals, 9001/9002 for optical ones.
- “Cultured crystals” in 3824.99 excludes optical elements—so even if shaped, if not for optics, it’s not 9001/9002.
- No priority rule is given; choose based on **primary function** or **intended use**