I have not used Blitz3D in a long time, but I think there is an entity flag to make an entity fullbright. You'd lose all lighting effects on the gem but it would be bright.
Your other option would be to set your texture blend mode to add (not the entity blend mode) which would cause the texture to be added to the flat color untextured, but shaded gem surface, instead of multiplied with it like it normally is. That might let you have some lighting.
But in neither case would you be able to adjust the glow. You'd either have to recreate the texture over and over to adjust it, or create an object with multiple layers of polygons and use special blending modes, and I can't remember how to do that or if it's even possible to make it work. Or maybe render the camera view multiple times with varying object alpha with one version being fullbright.
Your other option would be to set your texture blend mode to add (not the entity blend mode) which would cause the texture to be added to the flat color untextured, but shaded gem surface, instead of multiplied with it like it normally is. That might let you have some lighting.
But in neither case would you be able to adjust the glow. You'd either have to recreate the texture over and over to adjust it, or create an object with multiple layers of polygons and use special blending modes, and I can't remember how to do that or if it's even possible to make it work. Or maybe render the camera view multiple times with varying object alpha with one version being fullbright.