A gel pen uses ink in which pigment is suspended in a water-based gel. Because the ink is thick and opaque, it shows up more clearly on dark or slick surfaces than the typical inks used in ballpoint or felt tip pens. Gel pens can be used for many types of writing and illustration.
The general design of a gel pen is similar to that of a regular rollerball pen, with a barrel containing the writing mechanism and a cap, and a reservoir filled with ink. The barrels can be created in many different sizes and designs; some have finger grips of rubber or plastic. The size of the nib or pen tip ranges from 0.18 mm to 1.5 mm.
The main advantage of gel ink is its high viscosity, which supports a higher proportion of pigments in the medium. The pigments are typically copper phthalocyanine and iron oxides, and the gel is made up of water and biopolymers, such as xanthan gum and tragacanth gum, as well as some types of polyacrylate thickeners. The pigments are opaque, and gel pens are available in a rainbow of bright and pastel colours, as well as opalescent, metallic, and glittery colours which show up clearly on dark paper.
- Create a bolder line
- Smudge more, due to ink being wetter
- Have occasional skips, due to the ball being less evenly coated with ink
- Use more ink, leading to more frequent refills or replacements
- Are more likely to leak ink when the stopper fluid drains out
- May be able to write on smooth, non-absorbent materials
- Are less prone to bleed through to the other side of a piece of paper
- Create finer, more controlled lines
- Can create white or brightly coloured opaque lines on dark surfaces
- May be able to write on smooth materials
Paint markers use an oil-based paint/ink that is very opaque and overlays all other colours beneath it. Compared to paint markers, gel pens:
- Are much smaller, with finer tips
- Have no odour and do not require shaking to mix pigment with solvent
Another trait of gel ink is that it resists an analytical method commonly used to assess the potential age of ink for forensic purposes. The United States Secret Service has maintained the International Ink Library  for many decades. Because manufacturers change their ink formulas slightly from year to year, thin-layer chromatography (TLC) can be used on ink from traditional pens to trace the manufacturer and date of manufacture of most inks. The pigments in gel ink do not dissolve, and therefore cannot be analyzed with TLC.
- Schwartz, Debra A. (September 2001). "The Last Word: Just for the gel of it". Chemical Innovation 31 (9): IBC.
- LaPorte, Gerald; Arredondo, MD; McConnell, TS; Stephens, JC; Cantu, AA; Shaffer, DK (May 2006). "An Evaluation of Matching Unknown Writing Inks with the United States International Ink Library". Journal of Forensic Science 51 (3): 689–92. doi:10.1111/j.1556-4029.2006.00144.x. PMID 16696723.