What does Long John look like?

Quoting from BPv2 Player's Guide, page 217: "The purest ore is translucent, and in dim light it almost glows. Such pure nodules are rare and uniformly small. More commonly, the ore is found in larger nodular deposits intermixed with other mineral compounds and organics, giving the ore a milkier and grainier appearance. The specific gravity of purified xenosilicate is 6.8 with a hardness comparable to diamond. Its specific heat appears variable by grade with some types of ore being almost impossible to melt. Xenosilicates are found in large but isolated deposits, as nuggets or nodules of various sizes. Individual nodules usually mass less than .5 kg but there are unsubstantiated records of nodules in excess of 50 kg. Most nodules are spherical in shape with multiple radiating spurs or spines that can be over a meter in length. In some cases these spurs interconnect nodules implying that they somehow serve as catalysts for the deposition of new nodules."

True, but there's much more info on xenosilicates already provided than on other fictional ores in most sci-fi settings.
The word milky is used to describe xenosilicate wafers more than once and the ore is described as milkier when intermixed with other minerals. Also, if it was consistently of a particular eye-catching colour, I think that would get mentioned somewhere. Most minerals on Earth are white or colourless.

I think you won't break canon at all regardless of which colour you decided it has. I personally tend to go for various grades of translucency between crystal-clear and white/grey.