Larimar has been tentatively identified as pectolite (NaCa2Si3O8[OH]), although doubt remains among several mineralogists about this determination. Since the material is of volcanic origin, and it is often a replacement mineral, great variability exists in its composition. Undoubtedly there is some pectolite, but chemical analysis indicates minimal amounts of sodium. Some needle growth within the blue areas has high aluminum content, which is suggestive of natrolite.
Much of the material is calcium silicate with occasional native copper flecks. Recent finds include "scenic" pieces with red plumes (iron) on a blue background with white "clouds". Other samples contain pieces resembling thomsonites, some talc and several micro minerals.
The deposit where it is found is clearly volcanic, and gas pockets are filled with various minerals, with Larimar being most abundant filler. It exists in several shades of blue and green, from translucent to dark.
Much of the highest-quality Larimar appears to fill pockets created by the volatization of trees burned by volcanic activity. Naturally, this was a variable process, as some trees were more rapidly and completely burned than others. As a result, there are some pieces of carbon, some with partial Larimar replacement, and some with no indication of their organic origin.
Hardness varies, but most material is from 5 to 7 on the Mohs scale. The color sometimes produces reticulate patterns of blue and white and there is often a pronounced silky area. The beautiful shades of blue and green come from traces of copper.