The rock was then used to smash the glass display cases.
"I suspect it's probably frustration from them getting in and realising they weren't going to get much," Hansen said.
"So then they just thought I'll smash stuff and take my frustration out that way.
"We've been relatively lucky, it's just a mess from all the glass."
The orcish behaviour comes at what's usually the busiest time of year for the jeweller, as tourists flock to the region.
"The biggest people who miss out are the visitors to Nelson who come to see us because of our iconic status," Hansen said.
"Those who've only got a few hours, or a day, are going to miss out.
Staff are busy clearing up the smashed glass, and Hansen said the store planned to be open again by tomorrow.
The silver lining was that it showed their security systems were working.
"We're just a small business with a bunch of people trying to get on with our lives.
"We've been here more than 40 years, and once a decade someone might try to break the front windows.
"It's all on video, it's all safety-armoured glass, so we're happy with how it went."
Police would like to hear from anyone who was in the central Nelson area at the time of the burglary.
Any information can be provided to be police on 03 546 3840, or anonymously to Crimestoppers on 0800 555 111.