Ruby crystals appear as single crystals and clusters up to several centimeters at several of the sapphirine localities in the Fiskenaesset area. The color varies from rose-red through pigeon-blood red.   Fiskenaesset rubies have an unusually high chromium content (up to 3.0%) and are reportedly strongly fluorescent under LW UV light.

Most of the crystals are opaque but some are translucent/transparent but very rare.  This material is well suited for cabochons and the resulting piece is a wonder of colors - ranging from deep red, green, yellow, gold, white, black, and many colors in between.

The red porphryitic corundum (ruby) crystals are set in an amphibolite matrix dominated by green hornblende with minor biotite mica.  There is some accessory sphene.