This is the widest range 10 speed cassette I've ever seen. It's a 13-38. The first 5 cogs are chrome plated Cr-Mo. The 6 and 7 cogs sit on a separate aluminum spider. The 8 and 9 cogs are attached to another spider that is integrated with the 38t cog, which is alloy. This saves some weight and shouldn't affect durability since it won't incur very much wear.
For those of you who want to know the weight, you know who you are, shame on you. But we forgive you. It weighs in at just under 431 g. Happy now?