Mystery Gemstone ?

Any smart Gemmologists out there  who might have a clue as to the ID of this Gemstone ? I am uncertain.
   RI 2.65
  SG 3.19
  WT 5.08ct
  Dimensions: 10.27 x 9.15 x  5.08 (mm)
  Luster: Adamantine
  Colour. Lemony golden brown
  Inclusions.   Straight Bright thin cavities running parallel to each other,  some crossing at 90 degrees others with slightly hooked ends.
   Facet Edges: Sharp Ish with some minor chipping.
   UV display None.
No spectrum lines visible:
   Refraction: Single   (I think ....Possibly Anomalous double refraction.  
  Almost all light entering this stone through the table facet comes straight back out at ya

it's got a high RI. Moissanite? Maybe.

Maybe Moissanite
Moissanite is double refractive and this stone appears to be single refractive . In other words, I have not been able to see double refraction even through my 30X microscope although a couple of those inclusions appear to be doubled close in to the girdle....Maybe it has been cut to hide any double refraction when viewed through the table.
   The inclusions in this stone are easily visible with a 10x loupe and a few are naked eye visible.
I guess it is moissanite with those Hi refractive index & specific gravity readings.

The main problem is that the thin parallel inclusions look a lot like Growth Tubes which is common in Tourmaline and are not seen in Moissanite, so I think my RI readings are way of the track (calculated using a Travelling Microscope) so more likely this stone is Tourmaline.
When I calculate the weight of the stone from the dimensions it matches the figures for Moissanite and tourmaline so I need to obtain an accurate RI reading before putting a name to this stone.