“gleam.” This is the same Hebrew word as “eye” in Ezekiel 1:18. The word “eye” was used of things that sparkled or caught one’s attention.
“beryl.” The stone has also been proposed to be yellow topaz. There is really no way to be sure.
“as if one wheel was inside another wheel.” This is impossible to fully understand, because we have not been given enough information. Many interpretations are possible and many have been given.
It seems quite likely that each wheel was composed of two wheels that were placed at 90-degree angles. If that is the case, then each wheel was similar to the cherub beside it in that it could move straight ahead in four directions without turning. Just as each cherub had four faces and each face was looking in a different direction so that the cherub could move straight forward in different directions, a wheel composed of two wheels at 90-degree angles would allow God’s cherubim chariot to roll straight forward in four different directions.
Another possible interpretation is that the “wheel in a wheel” description could mean that each wheel had an inner hub and outer rim, and that gave the picture of a wheel inside a wheel. That kind of wheel construction was common in the ancient world and is a standard way of constructing wheels. However, it seems that the common nature of wheel construction using a hub, spokes, and rim argues against that being what Ezekiel saw, because he would likely then have said that he saw wheels next to the cherubim.