Gem experts please - is this a ruby, a garnet or something else?

Hi all,

I've posted this ring before:

bought as a ruby ring from a well-known antiquity dealer in uk about two years ago. I never doubt the authenticity of the stone, but as today I got a chance to test it with the cheapest conductivity tester, it indicates a stone with lower hardness- around 6-8 or something. It significantly differs from other rubies of various sizes which doesn't seem like a calibration issue. It contains these inclusions one would expect from ruby and garnet, as shown here:
it flashes as the ring turns and has bluish shine

and it does not glow under uv light.

I know it cannot be fully identified with only these, but what do you think I may have here? Do you think the 'ruby' is stone dead given the judgement of the tester? Any chance for the tester to be wrong? The stone does not really matter for this ring, but I just want to know more...