The difference between gel pen and flash pen is pen oil, but the material is different. According to the relevant information, the flash pen is a stupefied shape inside the pen, which makes the whole pen flash by reflecting and refracting the light source. Gel pen, that is, gel pen, is a kind of pen using the principle of ball bearing. The refill is filled with water-based or colloidal ink, which is very different from the ballpoint pen with oily ink. The viscosity of writing medium is between water and oil.