Welsh Slate has been sold as part of a £455m deal.
Construction firm Lagan Group - which includes the North Wales slate business - has been sold to the UK based Breedon Group.
Welsh Slate is the world’s leading supplier of high quality slate products and employs around 200 workers at a group of quarries in North Wales.
The main site is at Penrhyn Quarry in Bethesda.
Breedon said it expects to achieve annual cost synergies across the group of approximately £5 million by the third full year following completion.
A spokesman for Breedon said it saw Welsh Slate as a "core part" of the new acquisition.
He added: "We have a track record of investing in and developing the businesses that we buy."
Unite union said that staff at Welsh Slate will receive briefings this week on the sale.
Unite's regional officer Paddy McNaught said: "Breedon are a well established company with a good track record.
"We are hopeful this will be positive for Welsh Slate and the future of the sites in North Wales."
The business in its current format was bought from Alfred McAlpine in December 2007 for £31m who had since the 1960s expanded the business from its headquarters at Penrhyn Quarry to include operations at Blaenau Ffestiniog, Cwt-y-Bugail and Pen yr Orsedd.
The combination of Breedon and Lagan will create a leading independent construction materials group in the UK and Ireland.
