There is a table called INDEX_STATS that contains
what you are looking for; however it only contains
a single row of data at a time and you must populate it.

To populate it: validate index YOUR_INDEX_NAME;

To View: select * from INDEX_STATS;

Output:

-- Column Name Description
-- ----------------------- ----------------------------------------- HEIGHT height of the b-tree
-- BLOCKS blocks allocated to the segment
-- NAME name of the index
-- LF_ROWS number of leaf rows
-- LF_BLKS number of leaf blocks in the b-tree
-- LF_ROWS_LEN sum of the lengths of all the leaf rows
-- LF_BLK_LEN useable space in a leaf block
-- BR_ROWS number of branch rows
-- BR_BLKS number of branch blocks in the b-tree
-- BR_ROWS_LEN sum of the lengths of all the branch block rows
-- BR_BLK_LEN useable space in a branch block
-- DEL_LF_ROWS number of deleted leaf rows in the index
-- DEL_LF_ROWS_LEN total length of all deleted rows in the index
-- DISTINCT_KEYS number of distinct keys in the index
-- MOST_REPEATED_KEY how many times the most repeated key is repeated
-- BTREE_SPACE total space currently allocated in the b-tree
-- USED_SPACE total space that is currently being used
-- PCT_USED percent of space allocated that is being used
-- ROWS_PER_KEY average number of rows per distinct key
-- BLKS_GETS_PER_ACCESS Expected number of consistent mode block
-- gets per row. This assumes that a row
-- chosen at random from the table is being
-- searched for using the index