I could save myself the trouble, and you the time, and simply state that Diamine Graphite is an almost identical twin of Diamine Grey but there are actually a couple of very small (but key) differences.
I reviewed this ink, as I did the previous, in my Kaweco Sport Classic with broad nib and eyedropper conversion.
Bottle and pricing
This being Diamine, a bottle sells for around $12.95, and it holds 80ml / 1.7oz. That makes it $0.16 per milliliter, or very affordable, in other words.
Color and saturation
Jumping right into the differences between Grey and Graphite, there are two things here that separate the two.
First, Graphite is a darker ink, edging even closer to black. You can clearly tell it's still grey but feels black when wet, turning lighter as it dries up.
Second, the color isn't really a neutral grey. To my eye, there are faint traces of green/khaki in it. Is this unpleasant? Not at all, in my opinion. Now, if you're looking for a perfectly neutral dark grey, you might be better served elsewhere but I find this ink very attractive. In fact I think I prefer it to Diamine Grey.
Here's a better comparison between the three grey inks I've tested so far, from lightest to darkest: J Herbin Gris Nuage, Diamine Grey and Diamine Graphite:
Shading
Like its lighter sibling, Graphite shows some very nice shading, especially when used with a broad nib such as my Kaweco Sport.
Feathering
None that I could see.
Bleedthrough
Diamine Graphite is darker still, meaning that it ghosts a little bit more on cheap paper. It might even bleed a bit if you're using a nib with ample flow but that won't be an issue on good paper.
Flow, lubrication, and smoothness
Like its sibling, Diamine Graphite flows smooth as silk, and also wet, the way I like it.
Drying time
Drying times on various papers match, once again, those of Diamine Grey. About 5 seconds for cheap paper and 15 on Rhodia, but bear in mind that I used the broad nib which typically takes longer to dry.
Smearing when dry
None.
Water resistance
For a regular ink, Diamine Graphite shows surprisingly good water resistance. After 1 minute exposure under flowing water, some of it has washed off but most still remains, leaving the text legible. You might notice that some of the dark green undertones are exposed by the water.
Conclusion
Once again (that makes it twice in a row) I'm impressed by a grey Diamine ink. I find Graphite even better than Grey, maybe because it's more assertive, being darker. All the good qualities that I found in Diamine Grey are reflected here, with the added bonus (or disavantage, depending on your point of view) that it gets from the subtle green undertones. Personally I like that because it gives it personality. Having said that, I highly recommend it, provided you are looking for a darker grey ink and don't mind the fact that it is not entirely neutral.
Here are the two samples done on photocopy and Rhodia 80g paper, respectively.
