One member of the group is understood to have distracted staff while the others managed to seize a number of trays containing rings.
Gardai confirmed that the overall cost of the jewellery may run into "several thousand euro".
The incident took place just after 2pm yesterday afternoon at a shop on Castleblaney main street.
Officers were this weekend due to examine CCTV footage of the robbery.
Those involved entered the store posing as customers before seizing the items once the assistant's back was turned.
Anyone with information is being asked to contact Castleblaney Garda Station on (042) 9740668.
It is the second theft from a jewellery store in Castleblaney this year.