How is it possible to add Tag:geological=dyke - OpenStreetMap Wiki to the list of geological values of https://wiki.openstreetmap.org/wiki/Key%3Ageological
There’s one (spelt with an “i”) in use in OSM currently. At that sort of usage level, documentation is a bit irrelevant.
How would you be thinking about using it? It’s something that exists in three dimensions, but you might see it on a horizontal surface or a vertical one. What sort of examples would you map with this tag?
There are also other very rarely used geological tags like Tag:geological=hoodoo - OpenStreetMap Wiki and Tag:geological=columnar_jointing - OpenStreetMap Wiki
I would map walls (like you can see them in the images) as lines or areas. And vertical lines integrated into other rock formations as nodes (as seen from top). There are many examples which you can see in Wikipedia and Commons, especially in volcanic areas.
And for geological dyke there are already more examples. 
See overpass turbo
I have added how to map: Tag:geological=dyke - OpenStreetMap Wiki