This effect is called "silvering" and is caused by air between the embedded object and the resin.
The object must be dry and sealed before embedding in the resin. If the object isnt sealed, the air inside the object is released and clings to the object creating a thin layer of air around the object. Or uneven surfaces on an object trap the air and create this issue. If casting metals, the exothermic reaction of the resin can cause expansion of the metal, and as the cast cools, the metal contracts causing the air pocket.
I always seal any object with a clear coat and leave to dry for a couple of days to degas the finish applied. When Im ready to embed the object, I use a brush to coat the object with the clear catalyzed resin to eliminate air being trapped.
