The difference between gel pen and flash pen is pen oil, but the material is different. According to the query related information, the flash pen is a stupefied shape inside the pen, which reflects and refracts the light source to make the whole pen flash. Gel pen, namely gel pen, is a kind of pen that uses the principle of ball bearing. The pen core is filled with water-based or colloidal ink, which is very different from ballpoint pen with oil-based ink. The viscosity of writing medium is between water-based and oil-based.