Apologies – you are correct – the density of individual pebbles should be about equal to the Bulk Density divided by the Packing Density. So if the Packing density is 0.7 then the actual density of the pebbles would be 1.26/0.7 = 1.80.
Not good to rush from one thing to another on my part. And one should read what one has written before posting! Sorry for that – I will edit the above and put it right.
In fact, my earlier post talked about the angular nature of the pebbles/boulders and the lack of dust particles, and I speculated that one reason is that this could be that the material formed in a relatively high-temperature environment and that it could be rich in carbon including elemental carbon. So having a true density of 1.8 (not 1.0) and being very dark would fit with that scenario.