Gem raised C$900,000 in total and Mosman is putting up C$80,000 to give it a stake of 16.5%.
Mosman executive chairman John Barr met with several significant shareholders in Gem before making a decision and said he was happy with plans to extend its option in Angola to include areas with possible kimberlite pipes as well as alluvial areas.
Gem also intends to raise a further C$3mln.
Elsewhere, Mosman has recently completed a fieldwork programme at its two licences in the Amadeus Basin where it noted there had been a bid recently for one operator from bank Aussie Macquarie while a stake in Meerenie next to its EP 145 licence had been sold for A$52mln also to Macquarie.
In New Zealand, a farm-in partner is being sought at Murchison while the Petroluem Creek and Taramakau licences are to be relinquished.