Complex sentence: Pure silicon crystals are very rarely found in nature, but notable exceptions are crystals as large as to 0.3 mm across found during sampling gasses from Kudriavy volcano on the Kamchatka Peninsula.
Split into simpler sentences: Pure silicon crystals are very rarely found in nature. Notable example of pure silicon found in nature are silicon crystals up to 0.3 mm across, found during sampling gasses from Kudriavy volcano on the Kamchatka Peninsula.