Tremolite crystal in calcite
Tremolite:  dark gray, elongated crystal
Calcite:  white matrix, scraped away w/ sharp object

This is an interesting specimen of what appears to be dark gray tremolite {Ca2Mg5Si8O22(OH)2}. At first glance I thought it might be corundum, but its hardness was much lower than corundum. It also lacks the barrel- or cigar-shaped habit of corundum crystals, thin at the ends and thicker in the middle-- though that in itself is not an indicator, because some corundum crystals are straight-sided.

It is a solid, prismatic crystal that separates into smaller crystals at one end. While tremolite is fairly common in the Franklin marble as blebs and masses, it was only recently that we decided to look for crystals of it. Usually we end up getting sidetracked looking for fluorescent minerals such as diopside and norbergite... another time-consuming quest in itself.

From time to time we've also found what appear to be pseudomorphs of diopside after tremolite, in which brightly-fluorescent diopside has replaced bladed crystals of tremolite. These seem to be easier to find than actual tremolite crystals, at least in the areas we've looked.

Collected in 2000 during a F.O.M.S. field trip to the Lime Crest Quarry in Sparta, New Jersey. 

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