Since I recommended this ink to someone who was unhappy with Nitrogen from Organics studio, I thought I would try to showcase it properly which is rather difficult for a sheening ink.
This is Ostrich's Sea of Okinawa from their sheen line, it presents as a bright blue with red sheen. They are available in 18mL plastic bottles, or 30mL glass bottles. The glass bottles are a lot more expensive per unit of volume (so I stick to the smaller plastic ones).
The Sea of Okinawa is a very well behaved sheen, and depending on the pen and paper it can even be rather subdued: broad wet nibs show it as a bright blue, as it does from this Picasso 902 M. But in this Pilot Custom 74 F, it shows darker, with no shading and a rather flat sheen. This ink is very well behaved on a lot of papers and barely impacts ink flow in the pens I have tried it in. It is a bit of a nib creeper.
This ink can show tremendous shading and sheen on the right paper and in the right pen, and in that respect, it is rather picky, as it favors broader nibs to show shading and variation in sheen. The Waterman Exception even shows a haloing effect with this ink and its rather broad M nib, that I have not seen with anything [and I stand corrected on that]. It behaves well enough to be used as a daily, in work/professional setting, no problem, as basic copy paper will not show outlandish sheen.
(Oxford Essentials, 90g/m² Optik paper)
On this Oxford "Optik paper+", the Picasso 902 M shows some interesting effects, some very light haloing, though not as much as the Waterman Exception M.
(Oxford Essentials notebook, 80g/m² Optik paper+)
Nib creep on Pilot Custom 74 F
Mild haloing on 80g/m² Optik paper+ from a Picasso 902 M
Sheen from a drop of ink on 90g/m² Optik paper, ink name written with steel nib dip pen
On some papers (Clairefontaine's velum papers for instance), it shows a bright red metallic sheen, on some others (like the Optik paper and the Optik paper+) it shows a little more subtle, but I have not seen it feather or bleed though any of the paper I use, including the super cheap Action notebook (still with decent 80g/m² paper). White paper obviously makes the sheen more noticeable, but it does work well with a broad variety of papers nonetheless, it will just be less noticeable. I have also used this ink in a Picasso 916 with a SEF nib that's thinner than Pilot's EF steel nibs, and it caused zero issues in terms of flow, and still showed sheen.
I apologize for not showing the ink in the Waterman Exception after bringing it up, but I just filled it with some Emeraude de Chivor and I love this ink way too much to waste even a drop of it. If anyone is really curious I can add a sample at a later date, but it's really hard to capture haloing.
Edit: I forgot to mention that Ostrich also has a more saturated version of it (that I have not tried) called Aegian sea. But after my experience with their Glen of the sea, an insanely complexe extremely dark purple that looks black, feathers pink and sheens bright metallic green on anything, including toilet paper, I have been reluctant to try it. Because the Glen of the sea has an extremely hard time flowing in a fountain pen, it crusts up everywhere, smudges (pink) even when dry. Diluting it doesn't really help much, it shows less sheen, but not more color and doesn't even improve the flow.
Thanks again, SimplyGreg (I was the one not happy with Nitrogen). Appreciate it!
I hesitated to tag you, but I opted for the addition of the second part of the title instead.
Although Nitrogen may not be a complete write off, you could try diluting it with distilled water, not directly in the bottle it comes in, but in something else in case you mess up and dilute too much. From what I have read, I would start at 60/40 ink/water. But start as small as is practical for you, not even enough to fill up your eco, and then use it as a dip pen to see how much sheen you still get. Though it seems pretty far from the Sea of Okinawa, Nitrogen seems like a very light Cyan that is super saturated, so diluting it might change the underlying hue to something you won't like.
Yeah, I currently have a pen with about a 50% water-Nitrogen mix. It sheens less like that, but at least it isn't infuriating to use as a regular writing ink that way
doesn't hard-start if I stop writing for a handful of seconds.
I pretty much entirely use fountain pens for writing, not doing decorative signatures or whatnot. Being pretty is really just a fun plus for me; what's most-important is that it be pleasant to write with. Having the very-most-sheeniest of inks isn't really terribly important, just fun.
It shows up as maybe a medium-dark blue at the 50% dilution point for me. Not far off the Sea of Okinawa ink, somewhat darker.
This experience did inspire me to pick up a Opus 88 Jazz so that I have one eyedropper pen as well, to mix things directly into the pen with a syringe. The TWSBI Eco I did the water-Nitrogen mix in was approximate, and I wasted ink to do the dilution, dipping the thing in water to suck some up, given that the ink was already in the pen.