Foreign investment in mining of non-ferrous metal ores, except uranium and thorium ores, is allowed only through joint venture with Cambodian-owned investors. 
"Article 1: The following investment sectors shall be restricted on the basis of national security, social welfare, or economic needs as listed in the sensitive list for the opening of industries and for the granting of national treatment in the attached annex.
Annex 1 
[...] II. Industries Open with Restriction to Foreign Investors: [...] Mining of non-ferrous metal ores, except uranium and thorium ores [specifically: Exploitation of gemstones]: subject to local equity participation" (Sub-decree 80/ANK/BK on Measures Restricting Certain Investment Sectors, 27 August 1999, Art. 1 and Annex 1)
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"Art. 6: An Artisan Mining License may be issued only to persons of Khmer nationality for the purpose of conducting the exploration and exploitation of mineral resources by using locally available common instruments and their own labor or with the help of family with no more than 7 (seven) persons. [...]" (Law on Mineral Resource Management and Exploitation, Royal Decree  NS/RKM/0701/09 of 13 July 2001, Art. 6)
Sub-decree 80/ANK/BK on Measures Restricting Certain Investment Sectors, 27 August 1999, Art. 1 and Annex 1.
Law on Mineral Resource Management and Exploitation, Royal Decree  NS/RKM/0701/09 of 13 July 2001, Art. 6.
[<a href="https://www.wto.org/english/thewto_e/acc_e/khm_e/wtacckhm3a3_leg_4.pdf">Sub-decree</a>],[<a href="http://www.cambodiainvestment.gov.kh/law-on-mineral-resource-management-and-exploitation_010713.html">Law</a>]