Owning natural resources was considered gentlemanlike....

E.g. COAL - the fuel of the Industrial Revolution!

Check out these coal mining magnates:

The 2nd Marquess of Bute ("owned Cardiff" and the South Wales coal industry)

The 1st Earl of Durham (owned County Durham coal fields)

Mary Bowes, Countess of Strathmore and Kinghorne - County Durham coal mine heiress

The 4th Duke of Portland (his coal mines were in Scotland, but his residence in the Nottinghamshire Dukeries was bordered by neighbouring peers' coal mines.)

The 7th Duke of Devonshire (owned coal mines in Derbyshire)

A lot of peers invested in stocks, railroads and mining in the UK, the USA and various parts of the empire etc. Passive ownership of resources and investments was quite OK, it was just ungentlemanlike to actively be running and managing any business, with the exception of agriculture.

BTW owning silver mines and infrastructure (roads) were in the Middle Ages royal rights in several countries, so ownership of coal mines and railway stocks had something grand and regal about them.