In my opinion, you can diversify at these levels:

Local (by headquarters, by turnover)

By currency

By industry

By company size (influences volatility)

I try to have as little overlap as possible in several areas in order to keep my portfolio robust.

Example:

$DRO (+1.38%) and $PARRO (+3.53%) : Similar industry, so both are driven by the same news, but have different locations and currencies.

$CACI (+1.21%) Also has some correlation with the two, but is mainly dependent on the movements of the US military.

$8001 (-1.2%) As a boring anchor

$SL (+3.1%) As a "real" luxury play to profit from rising inequality and to have more euro/Italy in the portfolio.

$GRE (+1.41%) Because I see great potential in Greece, see old post

Strong GDP growth and shrinking debt, which is reflected in ytd performance.

If Trump plays around with the tariffs again, there will definitely be more to come. $GRE (+1.41%)

More on Greece's macroeconomic situation here.

Source: https://economy-finance.ec.europa.eu/document/download/2af158ef-d561-47c2-a0e0-5f689f1bac28_en?filename=ip309_en.pdf

If the deal goes through, it could give a boost to Greek market confidence.

It is offered 6.90 fur $EXAE (-0.68%) is offered.

$GRE (+1.41%)
$OPAP (+0.37%)
$MYTIL

https://finance.yahoo.com/news/euronext-talks-buy-athens-stock-152542880.html