These are faceted dichroic yellow/blue moissanites--depending on the orientation of the cut they can show blue, yellow, sparks of both or a blend to green.
Some of these stones have growth needles.
This is moissanite, also known as silicon carbide. It is an excellent gem, with incredible hardness (around 9.25-9.5) and greater brilliance and fire than a diamond. It is perfect for use in all kinds of jewelry.
Though it is well known in the gem world it is also highly valued in the world of science and industry. It is a semiconductor just like pure silicon but far more durable and heat-resistant, so it is used to make computer chips that need to endure high stress environments like in a car engine. Fortunately for us these uses require very exacting standards and sometimes crystals are rejected. These crystals had growth defects and atmospheric contamination, resulting in hollow needles and carbon clusters which ruined them for semiconductor usage but left enough clean zones (after extensive trimming) to facet brilliant gems.
Though the gem world sometimes thinks of it only as a diamond stand-in I prefer to think of it as a gem all its own. It has greater luster, more brilliance and fire and a cooler range of colors than diamonds do and I just find it more appealing visually.
