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The sand inside the glass and the sand outside are the same material, but the ones inside are darker.
Light Paths settings are all turned up
Max Bounces total : 802
glossy : 478
Transparency : 441
Transmission : 353
I made sure the base color of the glass is completely white.
I tried lowering the alpha value, but the glass reflection starts to become fuzzy and I get a problem where I'm able to see the inner edge of the glass because I made the glass solid.
//EDIT: I improved my glass material a little more, you'll find it at the end. Be aware that this is no overall solution, depending on the objects and scene other material setups will work better - this one is a glass material for things like hourglasses, snow globes, ships in a bottle etc. where other objects are inside a glass container. Different glass objects like e.g. windows in an interior scene may look better using the refined glass material mixed by Is Shadow Ray.
To get more light on objects inside a glass object, one way would be to disable the Clamping > Indirect Light in the Light Paths settings. I think the default value is 10, if you set it to 0 the clamping will be disabled. This will make the inside a little brighter, however this may cause more noise and fireflies at low sample rates.
Another problem is, that even if you have a bright sunlight which produces sharp shadows outside of glass objects, inside the shadows will be diffuse and soft. To change this, you have to tweak the glass material by mixing it with a Transparency BSDF and the Is Shadow Ray output of the Light Path node. This way the inside will get very bright. To reduce the brightness to a normal level, you can plug the Is Shadow Ray and Diffuse Depth output into a Math node set to Maximum. This is in my opinion the best compromise for a glass material (note that it doesn't matter if you use the Glass BSDF or the Principled BSDF with Transmission = 1). However, this will result in the glass having no shadow.
What I don't recommend is increasing the Max Bounces. Yes, there are differences in brightness between bounce values from 0 to 10 maybe, but higher values will make much difference in this case and only increase render times.
To see that and compare with other methods I mention above, I made an overview of the different settings I tried:
I tried a lot of Math nodes on the Light Path outputs and it seems it's easier than I thought: it's bright enough inside the glass, shadows are sharp and the glass still throws a shadow - the Ray Depth output is working quite well, just not the default output value, but plugged into a Math node set to Greater Than with 2 as Threshold. Only the refraction seems to be a bit different. Well, nobody is perfect...
If you want 100~ transparent or transmission bounces you must set Max Bounces to a high number as well, but that isn't your main problem.
After bouncing into a glass object, when the ray strikes a backface that hit is registered as Glossy. You need to increase Glossy bounces as well.
Without enough bounces, the ray is reported back as lost which gives a black color for that sample, darkening the overall result.
I solved it by unchecking shadows under ray visibility in the glasses objects menu. It appears that the inner darkness was caused by the glass casting a shadow onto the objects inside of it.
