Dictionaries:

Wikipedia - Den frie encyklopædi
Free online dictionary (in Danish) with information about rocks, minerals and others. The dictionary can be edited by everyone.

The Free Dictionary by Farlex 
Free online dictionary (in English) with information about rocks, minerals and others, including chemical formulas, geological environments and industrial usage.

Geologynet Online Geology Databases
Free online geology-databases (in English) with information about minerals and rocks including chemical formulas, crystal structures, physical properties, geological environments and industrial usage.

Geology-info:

Geologiportalen - Nationalkomiteen for Geologi
Webpage (in Danish) based on links to primarily Danish geology-relevant addresses with information about geology in general, mineralogy, fossils, sedimentology etc.

Greenland Minerals Occurrence Map - GEUS
GIS-map (in English) with a search function on the mineral deposits and the different geological environments in Greenland.

De Nationale Geologiske Undersøgelser for Danmark og Grønland (GEUS)
Webpage (in English) with focus on activities related to GEUS. That is, geological surveys, consulting, research and geological mapping primarily connected with activities in Denmark and Greenland.

Other links:

Bureau of Minerals and Petroleum
Webpage of the Bureau of Minerals and Petroleum (in English) with information in relation to mineral and hydrocarbon exploration and exploitation licences in Greenland. The webpage is primarily aimed at the mineral and hydrocarbon industry.

Naalakkersuisut - Ministry of Industry and Mineral Resources
Webpage of the Greenland Government (in English) with information about the competence, objectives and projects of the Bureau of Minerals and Petroleum as well as news about minerals and hydrocarbons in Greenland.

NunaGIS
Interactive map of Greenland (in English) with many layers of information. You can use NanuGIS if there is no map of the area where you found the rock at the post office. On the NunaGIS website you can zoom in and out, mark the spot and print the map. The map should be placed in the box with the rock, when you send it in.