He will be managing the firm's exploration activity on its newly acquired Timantti diamond project that lies within the highly prospective Karelian craton.
Spencer's extensive background working within the Karelian craton includes leading the discovery of the Grib diamond mine that generated revenue of US$340mln in 2016 and is the eighth-largest diamond mine in the world.
Spencer joined De Beers in 1966 and has been involved with exploration and deposit evaluation for gemstones and other commodities throughout his career.
As technical director of Peregrine Diamonds, he discovered the first kimberlites on the Pilbara craton in Western Australia in 1989.
Spencer created and raised the seed finance for Ilmari Exploration Oy to explore for gold, base metals and diamonds on the Karelian craton in Finland. Ilmari went public in 2000 and discovered the Lentiira kimberlite cluster in central Finland in 2003.
He said in a statement: "Having worked extensively in the Karelian craton throughout my career, I am excited to begin our exploration program at the highly advanced Timantti project, where we have begun ground geophysics followed by drilling."
Last month, Arctic Star Exploration Corp said it had started its diamond exploration program on Timantti in Finland.
The company said ground geophysics will be conducted over the area of the 243 hectare exploration permit, which contains the diamondiferous Black and White Wolf kimberlites.
The ground-based, magnetic, gravity and electro-magnetic surveys are designed to help define the shape and size of the Wolf kimberlites, as well as to detect possible new kimberlite-like anomalies within the area for drill testing.
Shares added 3.85% to C$0.14.