I practice Mahayana tradition so I hope you do not mind my comments from my perspective.
In my practice, I try not to have too much attachment with Buddha's relics (Chinese: 舍利, Sanskrit: śarīra). As long as I have Buddha's teachings in my heart and actions, that's the most important thing.
That being said, I have seen Buddha's relics a few times and they are truly fascinating. More crystal-like matter organically grow from the original relic, and they can vary in different form, colour and size. The Tibetan establishment across the street from where I live has relics from Buddha and his foremost students. I was very fascinated by Moggallana's relics as they were very dark, which was caused his unwholesome actions in the past and led to his tragic death being clubbed to death.
In Chinese Buddhism records diligent individuals (monks and lays) also leave behind relics. As they have completed the training in this life they leave their body behind, the relics withstand cremation and are collected by fellow Buddhists and honored for their accomplishment.
Relics are not exclusive to Buddhists. Before Buddha's time many Hindu sages also leave behind relics. Any beings who successfully transcends to the Realm of Form and beyond may leave behind relics. Therefore, relics may not be reliable evidence that one has liberated from Samsara.
Hope this is helpful. If there is anything I can improve with this reply, please kindly let me know :)