During a prolonged cold spell during this year's winter in the UK, I noticed some unusual ice crystals which appeared to be "growing" out of a stone in the garden (attached picture).  The temperature had been below 0 degrees for about 10 days but most of the frost had gone from the other stones around this particular one. 

The height of the ice crystals was around 1 cm with the overal size of the stone around 5 cm although the ice covering was very patchy.  Could a porous stone create this effect when the water inside freezes and is forced out of the pores...?