Different inks have different properties, including: sheen, shimmer, shading, etc, which is part of what makes inks so fascinating. I find shading and sheen to be interesting, and use them more than inks that don't have special properties. So far I haven't had much experience with shimmering inks, so I'll talk about them after I have had more time to experiment with shimmer.
The definition of sheen is a soft luster on a surface. Sheen in an ink is an appearance that the ink shines with a different color, that appears metallic or shiny. Sheen is different than shimmer, which is when metallic particles are added to the ink. An example:
On the left is Diamine Golden Sands, which is a dark yellow ink with gold shimmer. In the middle is a swab of J. Herbin Emerald of Chivor, which is a popular ink, and can be confusing because it is a sheening ink and a shimmering ink. It has a red sheen, and gold shimmer. On the right is Organics Studio Walden, which has the most sheen I have ever seen in an ink.
Sheening inks can be divided into five categories. For the purposes of reviewing ink, I evaluate an ink's sheen based on how it performs on Tomoe River paper. A lot of inks show sheen in large swabs or ink drops, but some of them only show sheen in large swabs, but don't show any sheen in writing. I consider these inks to have a tiny bit of sheen. Inks that show sheen when used for writing fall into the other four categories: low (shows sheen in 1-2 nib sizes), medium (3-4 nib sizes), high (5+ nib sizes), and mega. Mega sheeners are inks that sheen in 5+ nib sizes on other papers than Tomoe River, such as Organics Studio Walden, which has high sheen on every paper I tested it on, including Baron Fig Confidant paper, Leuchtturm 1917, and Rhodia.
I have only included inks I have already reviewed. Sheen is a popular topic in the fountain pen community, so I will keep this page current, updating it whenever I review an ink that has sheen. Click on a swab to be taken to a review and pictures of that ink.
Tiny sheen (sheens only in large swabs on Tomoe River paper):
Low sheen (sheens in 1-2 nib sizes on Tomoe River paper):
Medium sheen (sheens in 3-4 nib sizes on Tomoe River paper):
High sheen (sheens in 5+ nib sizes on Tomoe River paper):
Mega sheen (sheens in 5+ nib sizes on papers other than Tomoe River paper):
Other resources if you love sheen:
Yagan Kiely on his blog Macchiato Man talks about the difference between sheen and shimmer.
Brian Goulet, from Goulet Pens talks about his favorite sheening inks.
Disclaimer: All photos and opinions are my own. This page is based on my experience with sheen, and is not to be taken as fact.