After reading the given answers (as now), let me add mine :
If you plan to draw long lines for hanging your lights between pillars, the following is for you, else @isherwood answer should be the good way.
As a father, I always think about "where will this dam' child find a way to hang/climb ?". So, in order to let my daughter climb everywhere without risking some installation to fall, i tend to build really solid things.
Long lines will pull on the stone (traction force). You need to convert this into compression in order to avoid the rock to split.
This is achievable e.g. by adding a steel stripe (section e.g. 30x8mm) in the back of your stone, with a steel strapping.
These stripes must be strongly tightened to the ground and reaching the back of the strapping, facing the traction point.
By doing this, the bending on the steel will try to "crush" the stone pillar, thus creating a compression force instead of a traction one.
I Hope i did explain that in an understandable way.