[Braduz (OP)]

Thank you for your help

[Kryptid]

Do some tests on it. Measure its hardness by trying to scratch it with common materials like glass and steel. See if it has a streak when rubbed on paper. You can measure its density by first finding its weight and then dividing that by its volume (you can submerge it in a water-filled beaker and note the resulting increase in volume to determine this. Be sure there are no bubbles stuck to the rock).  See if it fluoresces under a black light. Reactions (or lack thereof) to acids and bases might help too.

[impyre]

Just a guess here, but I'm thinking probably mostly quartz. It looks as if it has spent some time in a river. Milky quartz can take on a glassy appearance when wet.