refixing decorative stones

stator

My house has ugly decorative stones at the edges with pebbledash on the rest of the walls.
Several decorative stones have falled off. They had been refixed back in with something, you can see the more recent stuff. But lots are falling off now.

I've decided to just fix them to the wall with some kind of pin or screw. The house is quite ugly, redoing completely at some point in the next 20 years, so don't want to waste money just redoing the decorative stones.
What sort of fixing would people recommend? Looking for something subtle, the hole will be visible once it's filled in, so a small hole is an important requirement.

Owain_Moneysaver

Drilling the 'stones' for a nail or screw may shatter them - or enough of them to make the exercise painful.

Some type of flexible tile adhesive / mortar adhesive would probably be best. This might be suitable depending on what the quoins and your walls are made of

stator

The walls are made of poured no-fines concrete.
No idea what the decorative stones are made from but they are all perfectly square so I'm thinking something moulded like cement
What's behind them is not secure, some kind of cemet or mortar. That's why I was going to bolt them to the wall.

Looks a bit like this
but most aren't corner pieces

I've got a few round the back of the house I can test drill I suppose, doesn't really matter if it shatters as the whole thing looks awful anyway. Would just prefer it looked awful on the wall rather than an awful hole where it used to be. Plus wondering about damp penetration where the stones are missing the pebbledash isn't thick