A selection query applied to a database often has the selection predicate imperfectly specified. We present a technique, called the Diamond-tree, for indexing fields to perform similarity-based retrieval, given some applicable measures of approximation. Typically, the number of features (or dimensions of similarity) is large, so that the search space has a high-dimensionality, and most traditional methods perform poorly. As a test case, we show how the Diamond-tree technique can be used to perform retrievals based on incorrectly or approximately specified values for string fields. Experimental results show that our method can respond to approximately match queries by examining a small portion (1% - 5%) of the database.