The charring of wood as a method of preservation is nothing new and was known to European and Mediterranean builders as well as Asians. I have for years charred the bases of deck posts and such as a rot preventative. The practice is mentioned in John Fitchen's classic book Building Construction Before Mechanization. Some of the samples of this shown above are a bit crude. The most beautiful finishes of this type are rather labor intensive as some of the char can be brushed back off to some extent to reveal wood grain. The planter shown below has been coated on the interior with linseed oil, blackjack and turpentine and then set alight with a roofer's torch. The finish is cooked into the wood producing a carbonized insect and decay resistant surface. It gets filled with soil and so the char will be seen only at the edges.

Look at 'Iron Ash'. Its a treated version of what you're after. Don't know anything about it but seen it recently on the web. Overall avoid Merbau as mentioned above, as well as being an imported 'sustainably' managed timber.... Use local product's when available as they're manufacturered to high standard's, supporting local job's, companies must meet tight regulation's, etc. unlike other countries....

This is the inside of the house. The bottom photo is the inside of that extension. The House is a Yellowy Creamy colour that is very dated and we want to change to a more neutral off white at a later date. I did think that a white or off white for the outside might look a bit too white and washed out with the white Brick. The Brick does have like charcoal flecks through it. So maybe that colour could be good to coordinate with.

Got my answer - non combustible material such as, natural stone - marble, granite or concrete etc.