-ling is always masculine (I would not call them diminutives, you cannot form them from many words)

-chen and -lein (diminutives) are always neuter, they can be formed from many nouns

-heit and -keit, and -ung are always feminine

I don't claim that the gender has logical meaning. But for derived words the rules typically hold (I would not be suprised if a few exceptions could be found)

> I assume you mean to only talk about "-chen"-suffixed diminutives right? [ there are other kinds of diminutive construction in German that aren't neutral - e.g. -ling , Zögling, takes masculine ]

Which is even more fun as -ling is generally seen as more objectifying than -chen, even though -chen is neutral and -ling is male.

Also -ling is not so much a diminutive but more of a suffix for Nominalisation (Zögling stems from 'ziehen' [to raise],and refers to someone who was raised, Widerling from 'widerlich' [despicable], thus a despicable person, Schreiberling from 'schreiben' [to write], a Writer], Frühling from 'früh' early [spring]). Gives a somewhat pejorative connotation at times.

The key misunderstanding here is of course that 'Genus' is not necessarily 'Sexus'.

[I'd like to acknowledge that my designation of "-ling" as functioning sometimes as a diminutive is a thing that might be debated for all the grounds given in this and in another comment, but I think it would make precisely nobody happy to pursue the matter further (while at the same time appreciating the points made, that do indeed give me pause)].