You need to specifiy a bit more info, e.g. like this
@prefix crop: <http://example.org/foo#> .
@prefix crops: <http://example.org/foo#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/TR/rdf-schema/> .
crop:AttributeValue a rdfs:Class . crop:SomeValue a rdfs:Class; rdfs:subClassOf crops:AttributeValue .
<http://example.org/foo#SomeValue/7> a crops:SomeValue .
crop:SomeValue a rdf:Property ; rdfs:range crops:SomeValue .
Replace the namespaces for crop and crops with the correct ones.
This would be the following in RDF/XML
<?xml version="1.0"?>
<rdf:RDF xmlns:rdfs="http://www.w3.org/TR/rdf-schema/" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:crops="http://example.org/foo#">
<rdfs:Class rdf:about="http://example.org/foo#SomeValue">
<rdfs:subClassOf>
<rdfs:Class rdf:about="http://example.org/foo#AttributeValue" />
</rdfs:subClassOf>
<rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property" />
<rdfs:range rdf:resource="http://example.org/foo#SomeValue" />
</rdfs:Class>
<crops:SomeValue rdf:about="http://example.org/foo#SomeValue/7" />
</rdf:RDF>
Here is an online tool for the conversion : http://www.rdfabout.com/demo/validator/