I have been trying to find decomposed granite here in Spain (which apparently Google translates as "granito descompuesto") but am yet to have any luck. I wanted to write this post to try and help anyone else in Spain who may also be looking for this, to be used as a growing medium for plants (like avocados!).
Several avocado experts have stated that decomposed granite (or DG) is a great growing medium - it has a very high mineral content - and this is preferred by avocados. It should be mixed with other mediums - peat moss was one recommendation, sand, charcoal and perlite being a few others that could be included in the mix (but some of those won't be essential, it depends on your own mix).
One issue I have is that its not clear what the name of DG is here in Spain - I have searched for "Granito Descompuesto" but this doesn't yield many good results.
I had discovered "Greda volcánica" but this seems to be a red rock and its not exactly clear if this will work as well as DG (my intuition says yes - it is probably also good - but the stones look quite large, more similar to gravel).
There are also other "Piedra Volcánicas" but again, many of these seem to have quite large piece sizes, whereas decomposed granite was far smaller (I imagine, because it has decomposed somewhat, and broken down in size).
Piedra pómez is another substrate I came across in my search - this seems quite close to DG and is described as being good for retaining nutrients, and being good for drainage. And this was sold by a plant nursery based in the Netherlands (which apparently ships to Spain) called SY Botanica. What confuses me is that the bag label (in English) is called "Bag of Pumice" - not decomposed granite (!) - so I don't know if this is quite the same material or not.
And then there is something known as Canto Rodado or Gravilla De Río - this does appear somewhat smaller, but the websites that sell this appear to be more landscapers or builders merchants. And again it isn't that fine (if it is being shown to be laid as a hard landscape in a front garden, I'm not convinced it is the right size for growing avocados!).
Finally, there is Piedra de lava triturado which is sold by Leroy Merlin - but again it is a red rock and looks quite large, again being used as a landscaping material. I like this option because I should be able to get hold of this locally - which I prefer to do, vs having to order from somewhere outside of Spain.
I am quite conscious of the environmental cost of buying in various materials, when my own reason for growing avocados is largely relating to the environment, and of course climate change.
And this is relevant not just for finding decomposed granite, but for any other good growing materials - including peat moss, which gardeners should really be trying to avoid using (the harvesting of peat moss can destroy wetland ecosystems, and it also acts as a carbon sink).
For me the answer here is to try and source materials locally and to use what you have available, when possible. You might not have the absolute perfect growing medium for your plants, but unless you are a high-end professional grower, perhaps you can settle for a less-than-perfect setup?
With that in mind - I will continue to try growing avocados on the soil I have available on my land, and will see if I can source rocks/gravel locally (I am conscious that even sourcing them from a local shop, might not mean they have been harvested (from the ground) locally... this is the challenge!).